um: generalize os_rcv_fd

Change os_rcv_fd() to os_rcv_fd_msg() that can more generally
receive any number of FDs in any kind of message.

Link: https://patch.msgid.link/20240702192118.40b78b2bfe4e.Ic6ec12d72630e5bcae1e597d6bd5c6f29f441563@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>