ext4: 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 ext3 patch from commit 44f4f729e7a1.

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

diff --cc fs/ext4/fsync.c
Simple merge