EX-7601 tgt: round write lock to chunk

EX-7601 tgt: round write lock to chunk

For unaligned writes, we need to round the write locking to
cover the any leading or trailing chunks.  We do this by
creating a local 'remote niobuf' to describe the rounded
range and doing the locking against that niobuf.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I2bdea620386ad229375647a0e2cc6180c9bd7aa6
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52961
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>