Use pmc_destroy_pmc_descriptor() to actually free the pmc, which is
consistent with pmc_destroy_owner_descriptor(). Also be sure to destroy
PMCs if a process exits or execs without explicitly releasing them.

Reviewed by:	bz, gnn
MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D958


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273236 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f