nfsd: Fix error return code in nfsd_file_cache_init()

Fix to return PTR_ERR() error code from the error handling case instead of
0 in function nfsd_file_cache_init(), as done elsewhere in this function.

Fixes: 65294c1f

("nfsd: add a new struct file caching facility to nfsd")
Signed-off-by: Huang Guobin <huangguobin4@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>