hwpmc: fix false runcount assertion in user callchain capture

[afscoelho_gmail.com]

Summary

pmc_capture_user_callchain() checks a PMC's runcount before walking
the user stack, but reads it without holding the spinlock that
protects it. hardclock() can run on the same CPU during the capture
and drop the runcount to zero in between, tripping the assertion and
panicking INVARIANTS kernels under load.

Move the check inside the existing spinlock, right where the code
already confirms the sample is still valid. No functional change on
kernels built without INVARIANTS.

Signed-off-by: Andre Silva <[e-mail]>
Sponsored by: AMD

[mhorne]

spinlock_enter() is weird, and a somewhat confusing name. I believe it is the disabling of interrupts that really prevents hardclock() from interfering. It is somewhat misleading to say that pm->pm_runcount is "protected" by a spinlock.

Regardless, code change LGTM.