devtmpfs: replace ->mount with ->get_tree in public instance

devtmpfs: replace ->mount with ->get_tree in public instance

To finalize mount API conversion, remove the ->mount op from the public
instance in favor of ->get_tree etc. Copy most ops from the underlying
ops vector (whether it's shmem or ramfs) and substitute our own
->get_tree which simply takes an extra reference on the existing internal
mount as before.

Thanks to Al for the fs_context_for_reconfigure() idea.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/20250205213931.74614-4-sandeen@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>