ring buffer: Propagate __rb_map_vma return value to caller

The return value from `__rb_map_vma()`, which rejects writable or
executable mappings (VM_WRITE, VM_EXEC, or !VM_MAYSHARE), was being
ignored. As a result the caller of `__rb_map_vma` always returned 0
even when the mapping had actually failed, allowing it to proceed
with an invalid VMA.

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20251008172516.20697-1-ankitkhushwaha.linux@gmail.com


Fixes: 117c3920 ("ring-buffer: Introducing ring-buffer mapping functions")
Reported-by:  <syzbot+ddc001b92c083dbf2b97@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?id=194151be8eaebd826005329b2e123aecae714bdb


Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>