LU-649 io: DIO doesn't need lock i_mutex

LU-649 io: DIO doesn't need lock i_mutex

There is a dead lock if a file is written with with normal IO, and
read with DIO at the mean time:
regular write: extent lock -> i_mutex
DIO read: i_mutex -> server take extent lock

i_mutex taken at DIO read is redundant, because server will take
extent lock to serialize with write and truncate. Removing it can
avoid the deadlock.

Change-Id: I280559cebae5d04e4fd943bf3fbe82d67e07834b
Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1829
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>