LU-13196 llite: Remove mutex on dio read

LU-13196 llite: Remove mutex on dio read

DIO reads in Lustre are protected by Lustre range locking
and do not need the inode mutex.  This code was removed
in LU-1669, the range lock was added for DIO reads in
LU-6227, and then the mutex was accidentally re-introduced
in LU-6260.

Remove it again.

Test-Parameters: envdefinitions=ONLY=16 testlist=sanityn,sanityn,sanityn,sanityn,sanityn,sanityn,sanityn,sanityn
Fixes: 98883bd3e2cc ("LU-6260 llite: add support for direct IO api changes")
Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: I5b3570e83a4b4ff36d9a22bc6bd3be5d5f991924
Reviewed-on: https://review.whamcloud.com/37419
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Shuichi Ihara <sihara@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>