LU-14093 ptlrpc: redefine uop_params_off to support gcc10

otherwise gcc10 complains about out-of-boundry access:

llog_swab.c: In function lustre_swab_update_ops:
llog_swab.c:137:46: error: array subscript 65534 is outside the bounds
of an interior zero-length array__u16[0] {aka short unsigned int[]}
[-Werror=zero-length-bounds]
  137 |    __swab16s(&uops->uops_op[i].uop_params_off[j]);

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I27981cbc79991cbd7a79cb90aec97bd1dc8b2f1b
Reviewed-on: https://review.whamcloud.com/40486
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>