jbd2: use GFP_NOFS for blkdev_issue_flush

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 <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Cc: [e-mail]

fs/jbd2/commit.c

···
 	if (commit_transaction->t_need_data_flush &&
 	    (journal->j_fs_dev != journal->j_dev) &&
 	    (journal->j_flags & JBD2_BARRIER))
-		blkdev_issue_flush(journal->j_fs_dev, GFP_KERNEL, NULL);
+		blkdev_issue_flush(journal->j_fs_dev, GFP_NOFS, NULL);
 
 	/* Done it all: now write the commit record asynchronously. */
 	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
···
 	if (JBD2_HAS_INCOMPAT_FEATURE(journal,
 				      JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT) &&
 	    journal->j_flags & JBD2_BARRIER) {
-		blkdev_issue_flush(journal->j_dev, GFP_KERNEL, NULL);
+		blkdev_issue_flush(journal->j_dev, GFP_NOFS, NULL);
 	}
 
 	if (err)