NFSD: Clean up splice actor

[ Upstream commit c7e0b781 ]

A few useful observations:

 - The value in @size is never modified.

 - splice_desc.len is an unsigned int, and so is xdr_buf.page_len.
   An implicit cast to size_t is unnecessary.

 - The computation of .page_len is the same in all three arms
   of the "if" statement, so hoist it out to make it clear that
   the operation is an unconditional invariant.

The resulting function is 18 bytes shorter on my system (-Os).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>