iomap: fix sub-page uptodate handling

bio completions can race when a page spans more than one file system
block.  Add a spinlock to synchronize marking the page uptodate.

Fixes: 9dc55f13

 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>