ext4: rename s_error_work to s_sb_upd_work

The most common use that s_error_work will get scheduled is now the
periodic update of the superblock.  So rename it to s_sb_upd_work.

Also rename the function flush_stashed_error_work() to
update_super_work().

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