NFSv4: Fix a compiler warning when CONFIG_NFS_V4_1 is undefined

Fix a compiler warning:
fs/nfs/nfs4proc.c:910:13: warning: 'nfs4_layoutget_release' defined but not used [-Wunused-function]
 static void nfs4_layoutget_release(void *calldata)
             ^~~~~~~~~~~~~~~~~~~~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>