xfs: mark xfs_buf_free as might_sleep()

xfs_buf_free can call vunmap, which can sleep.  The vunmap path is an
unlikely one, so add might_sleep to ensure calling xfs_buf_free from
atomic context gets caught more easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>