afs: Only update d_fsdata if different in afs_d_revalidate()

In the in-kernel afs filesystem, d_fsdata is set with the data version of
the parent directory.  afs_d_revalidate() will update this to the current
directory version, but it shouldn't do this if it the value it read from
d_fsdata is the same as no lock is held and cmpxchg() is not used.

Fix the code to only change the value if it is different from the current
directory version.

Fixes: 260a9803 ("[AFS]: Add "directory write" support.")
Signed-off-by: David Howells <dhowells@redhat.com>