jbd2: use GFP_NOFS for blkdev_issue_flush

jbd2: use GFP_NOFS for blkdev_issue_flush

commit 99aa78466777083255b876293e9e83dec7cd809a upstream.

flush request is issued in transaction commit code path, so looks using
GFP_KERNEL to allocate memory for flush request bio falls into the classic
deadlock issue.  I saw btrfs and dm get it right, but ext4, xfs and md are
using GFP.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>