When forking a child process with PMC_F_DESCENDANTS set in pmc_attach()

in the parent, we will inherit the pmcids but cannot execute any operations
on them in the child.  The reason for this is that pmc_find_pmc() only
tries to find the current process on the owners hash list, but given the
child does not own the attachment, we cannot find it.
Thus, in case the initial lookup fails, try to find the pmc_process state
affiliated with the child process, lookup the pmc from there using the
row index, and get the owner process from that pmc.
Then continue as normal and lookup the pmc context of the owner (process).

This allows us to call, e.g., pmc_start() in the child process before we
start the work there, but to collect the accumulated results later in
the parent.

Sponsored by:		DARPA,AFRL
Obtained from:		L41
Tested by:		rwatson, L41
MFC after:		4 weeks
Reviewed by:		gnn
Differential Revision:	https://reviews.freebsd.org/D2052

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