fs_parse: add uid & gid option option parsing helpers

Multiple filesystems take uid and gid as options, and the code to
create the ID from an integer and validate it is standard boilerplate
that can be moved into common helper functions, so do that for
consistency and less cut&paste.

This also helps avoid the buggy pattern noted by Seth Jenkins at
https://lore.kernel.org/lkml/CALxfFW4BXhEwxR0Q5LSkg-8Vb4r2MONKCcUCVioehXQKr35eHg@mail.gmail.com/


because uid/gid parsing will fail before any assignment in most
filesystems.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Link: https://lore.kernel.org/r/de859d0a-feb9-473d-a5e2-c195a3d47abb@redhat.com


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