mtx: move lockstat handling out of inline primitives

Lockstat requires checking if it is enabled and if so, calling a 6 argument
function. Further, determining whether to call it on unlock requires
pre-reading the lock value.

This is problematic in at least 3 ways:

• more branches in the hot path than necessary
• additional cacheline ping pong under contention
• bigger code

Instead, check first if lockstat handling is necessary and if so, just fall
back to regular locking routines. For this purpose a new macro is introduced
(LOCKSTAT_PROFILE_ENABLED).

LOCK_PROFILING uninlines all primitives. Fold in the current inline lock
variant into the _mtx_lock_flags to retain the support. With this change
the inline variants are not used when LOCK_PROFILING is defined and thus
can ignore its existence.

This results in:

text	   data	    bss	    dec	    hex	filename

22259667 1303208 4994976 28557851 1b3c21b kernel.orig
21797315 1303208 4994976 28095499 1acb40b kernel.patched

i.e. about 3% reduction in text size.

A remaining action is to remove spurious arguments for internal kernel
consumers.