MFC r362829:
The "pid" field in the LinuxKPI task struct is typically set to the thread ID
and not the process ID. Make sure the linux_task_exiting() function uses tdfind()
to lookup the BSD procedure structure pointer by the "pid" field, and only
fallback to pfind() when no match is found! This makes linux_task_exiting()
in line with the rest of the code.

Differential Revision: https://reviews.freebsd.org/D25509
Submitted by: Greg V <greg@unrelenting.technology>
Sponsored by: Mellanox Technologies