selsocket: handle sopoll() errors correctly

Without this change, unmounting smbfs filesystems with an INVARIANTS
kernel would panic after 10e64782ed59727e8c9fe4a5c7e17f497903c8eb.

Found by: markj
Reviewed by: markj, jhb
Obtained from: CheriBSD
MFC after: 3 days
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D32492