linuxkpi: improvements for linux_pid_task() and linux_get_pid_task().

Unify functions bodies.
Do not call tdfind() if pid is passed, and do not call pfind() if tid
is supplied.

Reviewed by: hselasky
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D25534