Fix a LOR between hwpmc and the kernel linker. When a system-wide
sampling mode PMC is allocated, hwpmc calls linker_hwpmc_list_objects()
while already holding an exclusive lock on pmc-sx lock. list_objects()
tries to acquire an exclusive lock on the kld_sx lock. When a KLD module
is loaded or unloaded successfully, kern_kld(un)load calls into the pmc
hook while already holding an exclusive lock on the kld_sx lock. Calling
the pmc hook requires acquiring a shared lock on the pmc-sx lock.

Fix this by only acquiring a shared lock on the kld_sx lock in
linker_hwpmc_list_objects(), and also downgrading to a shared lock on the
kld_sx lock in kern_kld(un)load before calling into the pmc hook. In
kern_kldload this required moving some modifications of the linker_file_t
to happen before calling into the pmc hook.

This fixes the deadlock by ensuring that the hwpmc -> list_objects() case
is always able to proceed. Without this patch, I was able to deadlock a
multicore system within minutes by constantly loading and unloading an KLD
module while I simultaneously started a sampling mode PMC in a loop.

MFC after: 1 month