afs: Remove afs_xosi

afs: Remove afs_xosi

Since OpenAFS 1.0, all platforms in libafs have a lock called
afs_xosi, which is acquired and released around calls like VOP_GETATTR
on cache files. However, this lock doesn't appear to protect anything;
on all platforms, the code that runs while the lock is held uses only
calls VOP_GETATTR and accesses local variables (aside from
afs_osi_cred, which we use similarly in many other places). The
purpose of the lock has never been documented, and is not mentioned at
all in the afs_rwlocks text file.

The comment by the afs_xosi lock declaration suggests that the lock
was originally introduced to protect access to 'tvattr', which perhaps
was a global variable in the past. All uses of 'tvattr' are local now,
though, so protecting access to it doesn't make any sense.

So, remove afs_xosi, to remove the unnecessary serialization of
VOP_GETATTR calls.

Change-Id: Ib3764600ae0155057361418c86b49a3507bdcd94
Reviewed-on: https://gerrit.openafs.org/13350
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>