nfsd: NFSD_FILE_KEY_INODE only needs to find GC'ed entries

Since v4 files are expected to be long-lived, there's little value in
closing them out of the cache when there is conflicting access.

Change the comparator to also match the gc value in the key. Change both
of the current users of that key to set the gc value in the key to
"true".

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