fs: move should_remove_suid()

Move the helper from inode.c to attr.c. This keeps the the core of the
set{g,u}id stripping logic in one place when we add follow-up changes.
It is the better place anyway, since should_remove_suid() returns
ATTR_KILL_S{G,U}ID flags.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>