- The sx assertions don't actually need the internal sx mutex lock, so
don't bother doing so.
- Add a new assertion SX_ASSERT_LOCKED() that asserts that either a
shared or exclusive lock should be held. This assertion should be used
instead of SX_ASSERT_SLOCKED() in almost all cases.
- Adjust some KASSERT()'s to include file and line information.
- Use the new witness_assert() function in the WITNESS case for sx slock
asserts to verify that the current thread actually owns a slock.
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