ext4: update direct I/O read lock pattern for IOCB_NOWAIT

This patch updates the lock pattern in ext4_direct_IO_read() to not
block on inode lock in cases of IOCB_NOWAIT direct I/O reads. The
locking condition implemented here is similar to that of 942491c9
("xfs: fix AIM7 regression").

Fixes: 16c54688

 ("ext4: Allow parallel DIO reads")
Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/c5d5e759f91747359fbd2c6f9a36240cf75ad79f.1572949325.git.mbobrowski@mbobrowski.org


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