nfsd: NFSD_FAULT_INJECTION depends on DEBUG_FS

NFSD_FAULT_INJECTION depends on DEBUG_FS, otherwise the debugfs_create_*
interface may return unexpected error -ENODEV, and cause system crash.

Signed-off-by: Chengyu Song <csong84@gatech.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>