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.

Reviewed-on: https://gerrit.openafs.org/13350
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit d75bc6370f625479a67c7c0a50cce23c4d4a4ce5)

Change-Id: I33dc7935872cf071c4221d46cbb458e1f77185a2
Reviewed-on: https://gerrit.openafs.org/13529
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>