RDMA/srp: Document srp_parse_in() arguments

[ Upstream commit e37df2d5b569390e3b80ebed9a73fd5b9dcda010 ]

This patch avoids that a warning is reported when building with W=1.

Cc: Sergey Gorenko <sergeygo@mellanox.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>