Fix assertion, &uc->uc_busy is never zero, the intent is to test the
uc_busy value, and not its address [1].

Remove the single use of the macro, write KASSERT() explicitely in the
code of umtxq_sleep_pi().

Submitted by:	Eric van Gyzen <eric@vangyzen.net> [1]
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274478 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f