[Virtio-fs] [virtiofsd] MR opened: xattr mapping add a new type "unsupported"

[virtiofs-bot at sinrega.org]

This is ported from the C version.

Right now for xattr remapping, we support types of "prefix", "ok" or
"bad". Type "bad" returns -EPERM on getxattr()/setxattr() and hides xattr in
listxattr().

A new semantics is needed where if an xattr is unsupported, then
getxattr()/setxattr() return -ENOTSUP and listxattr() should hide
the xattr. And, for getxattr(), when the mapping code returns -EPERM,
for type "bad", getxattr() code converts it to -ENODATA.

This is needed to simulate that security.selinux is not
supported by virtiofs filesystem and in that case client falls back
to some default label specified by policy.

So add a new type "unsupported" which returns -ENOTSUP on getxattr()
and setxattr() and hides xattrs in listxattr().

For example, one can use following mapping rule to not support
security.selinux xattr and allow others.

"-o xattrmap=/unsupported/all/security.selinux/security.selinux//ok/all///"
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/67