xfs: fix IOCB_NOWAIT handling in xfs_file_dio_aio_read

Direct I/O reads can also be used with RWF_NOWAIT & co.  Fix the inode
locking in xfs_file_dio_aio_read to take IOCB_NOWAIT into account.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>