spinlock: macroize assert_spin_locked to avoid bug.h dependency

In spinlock_api_smp.h we find a define for assert_raw_spin_locked
[which uses BUG_ON].   Then assert_spin_locked (as an inline) uses
it, meaning we need bug.h  But rather than put linux/bug.h in such
a highly used file like spinlock.h, we can just make the un-raw
version also a macro.  Then the required bug.h presence is limited
just to those few files who are actually doing the assert testing.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
CC: Thomas Gleixner <tglx@linutronix.de>