LU-12811 ptlrpc: pass buffer size to the swabbing functions

LU-12811 ptlrpc: pass buffer size to the swabbing functions

By adding a separate rmf_swab_len() function pointer to
req_msg_field, the buffer size can be passed to the swabbing
functions, e.g. lustre_swab_fiemap() in this patch, to avoid
invalid access, especially for small buffer.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I997e6a828f2f1cdfdb8a5fa241fa43ca0ae3677e
Reviewed-on: https://review.whamcloud.com/36435
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>