afs: Stop looking for dcaches on Get*DSlot errors

afs: Stop looking for dcaches on Get*DSlot errors

In various places in the code, we'll be looking for a dslot, calling
afs_GetValidDSlot (or afs_GetUnusedDSlot) in a loop. In a few places,
we currently keep looking for the dslot when we get an error back,
since afs_GetValidDSlot may return successfully for other slots, and
we might find the dslot we're looking for.

This behavior was introduced in a few commits, including:

- commit 2679af76 (afs: Traverse discard/free dslot list if errors)
- commit 00fd34a6 (afs: Handle easy GetValidDSlot errors)
- commit 9a558660 (afs: Cope with afs_GetValidDSlot errors)

This behavior means that if afs_GetValidDSlot/afs_GetUnusedDSlot
returns an error for a particular dcache slot, but other slots are
okay, then we may still find the dcache we're looking for.

However, by far the most common reason that
afs_GetValidDSlot/afs_GetUnusedDSlot fails is because our disk cache
is completely unusable; it is very rare that only a few slots cannot
be used, but others are fine (this would mean that the disk cache was
corrupted in oddly specific ways, or there are small isolated errors
in the underlying disk). So continuing the dcache search in these
situations is not very useful.

On Linux, this is most commonly seen by the underlying disk cache i/o
calls returning -EINTR, which can happen if a SIGKILL signal is
pending for the current process when we try to do the i/o. In this
situation, all attempts to read in a dslot from disk will fail; trying
other slots or waiting will not improve the situation. Depending on
which specific code path encounters an afs_Get*DSlot error, we can
then flood the log with "disk cache read error in CacheItems" messages
emitted from afs_UFSGetDSlot, since we keep calling afs_Get*DSlot in
our loop.

The worst offender of this is usually afs_GetDSlotFromList via
afs_AllocDCache, since we end up calling afs_GetUnusedDSlot for every
single dslot in the free and discard lists. However, our other call
sites that are looking for dcaches for a specific file can still
generate quite a few of these messages, since we'll end up calling
afs_GetValidDSlot for every slot in a dcache hash chain.

So to avoid flooding the log in these situations, change most callers
of afs_GetValidDSlot and afs_GetUnusedDSlot to stop on the first
error, and act like we never found a dcache that we were looking for.

This commit also adjusts one caller in afs_ProcessOpCreate, which was
not handling errors from afs_GetValidDSlot at all, and changes
FlushVolumeData to be able to return error codes.

Change-Id: I3047da690d39c000ef59dfc0ad526ecc5e382104
Reviewed-on: https://gerrit.openafs.org/13034
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>