ovl: strict upper fs requirements for remote upper fs

Overlayfs works sub-optimally with upper fs that has no xattr/d_type/
RENAME_WHITEOUT support. We should basically deprecate support for those
filesystems, but so far, we only issue a warning and don't fail the mount
for the sake of backward compat.  Some features are already being disabled
with no xattr support.

For newly supported remote upper fs, we do not need to worry about backward
compatibility, so we can fail the mount if upper fs is a sub-optimal
filesystem.

This reduces the in-tree remote filesystems supported as upper to just
FUSE, for which the remote upper fs support was added.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

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