[mips] make UMTX_CHAINS configurable at compile time.

The default (512) wastes quite a bit of space which doesn't really buy
us much on highly embedded systems which don't take a lot of locks in
parallel.

This makes it at least build time configurable so people can experiment.

sys/conf/options

@@ -220,6 +220,7 @@ SYSVSHM		opt_sysvipc.h
 SW_WATCHDOG	opt_watchdog.h
 TURNSTILE_PROFILING
 UMTX_PROFILING
+UMTX_CHAINS	opt_global.h
 VERBOSE_SYSINIT
 
 # POSIX kernel options
@@ -996,3 +997,4 @@ GPIO_SPI_DEBUG	opt_gpio.h
 EVDEV_SUPPORT	opt_evdev.h
 EVDEV_DEBUG	opt_evdev.h
 UINPUT_DEBUG	opt_evdev.h
+

sys/kern/kern_umtx.c

@@ -198,7 +198,9 @@ struct umtxq_chain {
 			 PRI_MAX_TIMESHARE : (td)->td_user_pri)
 
 #define	GOLDEN_RATIO_PRIME	2654404609U
+#ifndef	UMTX_CHAINS
 #define	UMTX_CHAINS		512
+#endif
 #define	UMTX_SHIFTS		(__WORD_BIT - 9)
 
 #define	GET_SHARE(flags)	\