ext4: track writeback errors using the generic tracking infrastructure

We already using mapping_set_error() in fs/ext4/page_io.c, so all we
need to do is to use file_check_and_advance_wb_err() when handling
fsync() requests in ext4_sync_file().

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