ext4: don't leave PageWriteback set after memory failure

ext4: don't leave PageWriteback set after memory failure

In ext4_bio_write_page(), if the memory allocation for the struct
ext4_io_page fails, it returns with the page's PageWriteback flag set.
This will end up causing the page not to skip writeback in
WB_SYNC_NONE mode, and in WB_SYNC_ALL mode (i.e., on a sync, fsync, or
umount) the writeback daemon will get stuck forever on the
wait_on_page_writeback() function in write_cache_pages_da().

Or, if journalling is enabled and the file gets deleted, it the
journal thread can get stuck in journal_finish_inode_data_buffers()
call to filemap_fdatawait().

Another place where things can get hung up is in
truncate_inode_pages(), called out of ext4_evict_inode().

Fix this by not setting PageWriteback until after we have successfully
allocated the struct ext4_io_page.

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