xfs: change kmem_free to use generic kvfree()

Change kmem_free to use kvfree() generic function, remove the
duplicated code.

Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --cc fs/xfs/kmem.c
Simple merge

diff --cc fs/xfs/kmem.h
Simple merge