NFSD: Clean up SPLICE_OK in nfsd4_encode_read()

[ Upstream commit c738b218 ]

Do the test_bit() once -- this reduces the number of locked-bus
operations and makes the function a little easier to read.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>