internal: add may_write_xattr()

Split out the generic checks whether an inode allows writing xattrs. Since
security.* and system.* xattrs don't have any restrictions and we're going
to split out posix acls into a dedicated api we will use this helper to
check whether we can write posix acls.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>