LU-12443 ptlrpc: fix reply buffers shrinking and growing

LU-12443 ptlrpc: fix reply buffers shrinking and growing

The req_capsule_shrink() doesn't update capsule itself with
new buffer lenghts after the shrinking. Usually it is not
needed because reply is packed already. But if reply buffers
are re-allocated by req_capsule_server_grow() then non-updated
lenghts from capsule are used causing bigger reply message.
That may cause client buffer re-allocation with resend.

Patch does the following:
- update capsule lenght after the shrinking
- introduce lustre_grow_msg() to grow msg field in-place
- update req_capsule_server_grow() to use generic
  lustre_grow_msg() and make it able to grow reply without
  re-allocation if reply buffer is big enough already
- update sanity test 271f to use bigger file size to exceed
  current maximum reply buffer size allocated on client.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I154c55d98f41406d0c932c7e8705e0ecf3dfa935
Reviewed-on: https://review.whamcloud.com/35243
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>