FRV: ip_fast_csum() requires a memory clobber on its inline asm

ip_fast_csum() requires a memory clobber on its inline asm as it accesses
memory in a fashion that gcc can't predict.

The GCC manual says:

 If your assembler instructions access memory in an unpredictable
 fashion, add `memory' to the list of clobbered registers.  This will
 cause GCC to not keep memory values cached in registers across the
 assembler instruction and not optimize stores or loads to that memory.

The bug hasn't been noticed in FRV, but it has been seen in PA-RISC.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>