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 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

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