UBUNTU: SAUCE: (namespace) posix_acl: Export posix_acl_fix_xattr_userns() to modules

UBUNTU: SAUCE: (namespace) posix_acl: Export posix_acl_fix_xattr_userns() to modules

Fuse will make use of this function to provide backwards-
compatible acl support when proper posix acl support is added.
Add a check to return immediately if the to and from namespaces
are the same, and remove equivalent checks from its callers.

Also return an error code to indicate to callers whether or not
the conversion of the id between the user namespaces was
successful. For a valid xattr the id will continue to be changed
regardless to maintain the current behaviour for existing
callers, so they do not require updates to handle failed
conversions.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>