vfs: Create __block_page_mkwrite() helper passing error values back

Create __block_page_mkwrite() helper which does all what block_page_mkwrite()
does except that it passes back errors from __block_write_begin /
block_commit_write calls.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>