dm bufio: change __GFP_IO to __GFP_FS in shrinker callbacks

dm bufio: change __GFP_IO to __GFP_FS in shrinker callbacks

commit 9d28eb12447ee08bb5d1e8bb3195cf20e1ecd1c0 upstream.

The shrinker uses gfp flags to indicate what kind of operation can the
driver wait for. If __GFP_IO flag is present, the driver can wait for
block I/O operations, if __GFP_FS flag is present, the driver can wait on
operations involving the filesystem.

dm-bufio tested for __GFP_IO. However, dm-bufio can run on a loop block
device that makes calls into the filesystem. If __GFP_IO is present and
__GFP_FS isn't, dm-bufio could still block on filesystem operations if it
runs on a loop block device.

The change from __GFP_IO to __GFP_FS supposedly fixes one observed (though
unreproducible) deadlock involving dm-bufio and loop device.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[lizf: Backported to 3.4:
 - drop changes to dm_bufio_shrink_scan() and dm_bufio_shrink_count()
 - change __GFP_IO to __GFP_FS in shrink()]
Signed-off-by: Zefan Li <lizefan@huawei.com>