nfsd: simplify test_bit return in NFSD_FILE_KEY_FULL comparator

[ Upstream commit d69b8dbf ]

test_bit returns bool, so we can just compare the result of that to the
key->gc value without the "!!".

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