jbd2: check return value of blkdev_issue_flush()

blkdev_issue_flush() can fail; make sure the error gets properly
propagated.

This is a port of the equivalent jbd patch from commit 349ecd6a3c0e.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --cc fs/jbd2/recovery.c
Simple merge