When creation of the v_pollinfo raced and our instance of vpollinfo
must be destroyed, knlist_clear() and seldrain() calls could be
avoided, since vpollinfo was not used.  More, the knlist_clear()
calling protocol requires the knlist locked, which is not true at the
call site.

Split the destruction into the helper destroy_vpollinfo_free(), and
call it when raced, instead of destroy_vpollinfo().

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:   3 days