afs: Fix AFS file locking to allow fine grained locks

afs: Fix AFS file locking to allow fine grained locks

[ Upstream commit 68ce801ffd82e72d5005ab5458e8b9e59f24d9cc ]

Fix AFS file locking to allow fine grained locks as some applications, such
as firefox, won't work if they can't take such locks on certain state files
- thereby preventing the use of kAFS to distribute a home directory.

Note that this cannot be made completely functional as the protocol only
has provision for whole-file locks, so there exists the possibility of a
process deadlocking itself by getting a partial read-lock on a file first
and then trying to get a non-overlapping write-lock - but we got the
server's read lock with the first lock, so we're now stuck.

OpenAFS solves this by just granting any partial-range lock directly
without consulting the server - and hoping there's no remote collision.  I
want to implement that in a separate patch and it requires a bit more
thought.

Fixes: 8d6c554126b8 ("AFS: implement file locking")
Reported-by: Jonathan Billings <jsbillings@jsbillings.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>