[SCSI] libfc: call ddp setup for only FCP reads to avoid accessing junk fsp pointer

Adds check to call fc_fcp_ddp_setup for only FCP read cmds to avoid
accessing junk fsp pointer at least in ESX since non FCP frame had
junk fsp value, though fsp is implicitly initialized to null
by __alloc_skb but with this patch no more relying on fsp
initialized to null value and hitting junk fsp ptr access.

Removes fsp pointer checking in fc_fcp_ddp_setup as this is not
needed any more since its only caller for FCP read will always
have a valid fsp.

Reported by: Frank Zhang <frank_1.zhang@intel.com>
Reported by: Rob Love <robert.w.love@intel.com>
Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>