[XFS] Make xfsbufd threads freezable

Fix breakage caused by commit 83144186


that did not introduce the necessary call to set_freezable() in
xfs/linux-2.6/xfs_buf.c .

SGI-PV: 974224
SGI-Modid: xfs-linux-melb:xfs-kern:30203a

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>