NFSD: Rename nfsd_reply_cache_alloc()

[ Upstream commit ff0d1693 ]

For readability, rename to match the other helpers.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Stable-dep-of: 4b148854 ("nfsd: make all of the nfsd stats per-network namespace")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>