afs: Return memcache allocation errors

afs: Return memcache allocation errors

During cache initialization, we can fail to allocate our dcache
entries for memcache. Currently when this happens, we just log a
message and try to disable dcache access. However, this results in at
least one code path that causes a panic anyway during startup, since
afs_CacheTruncateDaemon will try to trim the cache, and afs_GetDownD
will call afs_MemGetDSlot, and we cannot find the given dslot.

To avoid this, change our cache initialization to return an error,
instead of trying to continue without a functional dcache. This causes
afs_dcacheInit to return an error in this case, and by extension
afs_CacheInit and the AFSOP_CACHEINIT syscall. Also change afsd to
actually detect errors from AFSOP_CACHEINIT, and to bail out when it
does.

Thanks to gsgatlin@ncsu.edu for reporting the relevant panic.

Change-Id: Ic89ff9638201faae6c4399a2344d4da3e251d537
Reviewed-on: https://gerrit.openafs.org/13273
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>