fs: use must_set_pos()

fs: use must_set_pos()

Make generic_file_llseek_size() use must_set_pos(). We'll use
must_set_pos() in another helper in a minutes. Remove __maybe_unused
from must_set_pos() now that it's used.

Link: https://lore.kernel.org/r/20240830-vfs-file-f_version-v1-7-6d3e4816aa7b@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>