uprobes: make uprobe_register() return struct uprobe *

This way uprobe_unregister() and uprobe_apply() can use "struct uprobe *"
rather than inode + offset. This simplifies the code and allows to avoid
the unnecessary find_uprobe() + put_uprobe() in these functions.

TODO: uprobe_unregister() still needs get_uprobe/put_uprobe to ensure that
this uprobe can't be freed before up_write(&uprobe->register_rwsem).

Co-developed-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240801132734.GA8803@redhat.com