fuse: support idmapped getattr inode op

We have to:
- pass an idmapping to the generic_fillattr()
to properly handle UIG/GID mapping for the userspace.
- pass -/- to fuse_fillattr() (analog of generic_fillattr() in fuse).

Difference between these two is that generic_fillattr() takes all the
stat() data from the inode directly, while fuse_fillattr() codepath takes a
fresh data just from the userspace reply on the FUSE_GETATTR request.

In some cases we can just pass &nop_mnt_idmap, because idmapping won't be
used in these codepaths. For example, when 3rd argument of
fuse_do_getattr() is NULL then idmap argument is not used.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>