mbuf: don't include lock.h conditionally

Using keywords from opt_global.h in the system headers allows to
create cryptic kernel build failures, that depend on the options
used in the kernel config, very hard to debug and understand.

Fixes:	063d811465

sys/sys/mbuf.h

@@ -42,12 +42,7 @@
 #include <sys/systm.h>
 #include <sys/refcount.h>
 #include <vm/uma.h>
-#ifdef WITNESS
-#include <sys/lock.h>
-#endif
-#endif
 
-#ifdef _KERNEL
 #include <sys/sdt.h>
 
 #define	MBUF_PROBE1(probe, arg0)					\
@@ -791,15 +786,11 @@ m_epg_pagelen(const struct mbuf *m, int pidx, int pgoff)
 #ifdef _KERNEL
 union if_snd_tag_alloc_params;
 
-#ifdef WITNESS
 #define	MBUF_CHECKSLEEP(how) do {					\
 	if (how == M_WAITOK)						\
 		WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,		\
 		    "Sleeping in \"%s\"", __func__);			\
 } while (0)
-#else
-#define	MBUF_CHECKSLEEP(how) do {} while (0)
-#endif
 
 /*
  * Network buffer allocation API