NFS: Fix fscache super_cookie allocation

Commit f2aedb71 ("NFS: Add fs_context support.") reworked
NFS mount code paths for fs_context support which included
super_block initialization.  In the process there was an extra
return left in the code and so we never call
nfs_fscache_get_super_cookie even if 'fsc' is given on as mount
option.  In addition, there is an extra check inside
nfs_fscache_get_super_cookie for the NFS_OPTION_FSCACHE which
is unnecessary since the only caller nfs_get_cache_cookie
checks this flag.

Fixes: f2aedb71

 ("NFS: Add fs_context support.")
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>