configfs: Add CONFIGFS_ATTR_PERM helper

This new helper allows creating rw files with custom
permissions.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20250420104726.2963750-1-richard@nod.at


Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>