sysctl extern cleanup: sg

Extern declarations in sysctl.c should be moved to their own header file,
and then include them in relavant .c files.

Move sg_big_buff extern declaration to scsi/sg.h

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Acked-by: Doug Gilbert <dgilbert@interlog.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>