selsocket: handle sopoll() errors correctly

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

PR: 253079
Found by: markj
Reviewed by: markj, jhb
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D32492

(cherry picked from commit 04c91ac48ad13ce0d1392cedbd69c2c0223d206f)