[SCSI] fcoe, libfc: use single frame allocation API

Cleans up frame allocation APIs to have just single fc_frame_alloc API.

Removes _fc_frame_alloc, renames __fc_frame_alloc to _fc_frame_alloc.

Modifies fc_fcp_send_data for removed _fc_frame_alloc, fc_fcp_send_data
was the only user of removed _fc_frame_alloc.

Also Adds check in fc_frame_alloc to do mod by 4 for only non-zero
len value.

This patch is prep work to fix can_queue reducing in next patch.
Single fc_frame_alloc API helps in fixing can_queue reducing in
next patch.

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>