fuse: allow nanosecond granularity

Derrik Pates reports that an utimensat with a NULL argument results in the
current time being sent from the kernel with 1 second granularity.

Reported-by: Derrik Pates <demon@now.ai>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
fuse: don't WARN when nlink is zero

commit dfca7cebc2679f3d129f8e680a8f199a7ad16e38 upstream.

drop_nlink() warns if nlink is already zero.  This is triggerable by a buggy
userspace filesystem.  The cure, I think, is worse than the disease so disable
the warning.

Reported-by: Tero Roponen <tero.roponen@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>