uprobes: simplify error handling for alloc_uprobe()

Return -ENOMEM instead of NULL, which makes caller's error handling just
a touch simpler.

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: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240801132719.GA8788@redhat.com