[SCSI] srp.h: avoid padding of structs

Several structs in <scsi/srp.h> get padded to a multiple of 8 bytes on
64-bit architectures and end up with a size that does not match the
definition in the SRP spec:

                                     SRP spec     64-bit
    sizeof (struct indirect_buf)        20          24
    sizeof (struct srp_login_rsp)       52          56
    sizeof (struct srp_rsp)             36          40

Fix this by adding __attribute__((packed)) to the offending structs.

Problem pointed out by Arne Redlich <arne.redlich@xiranet.com>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>