six locks: Simplify dispatch

six locks: Simplify dispatch

Originally, we used inlining/flattening to cause the compiler to
generate different versions of lock/trylock/relock/unlock for each lock
type - read, intent, and write. This made the individual functions
smaller and let the compiler eliminate table lookups: however, as the
code has gotten more complicated these optimizations have gotten less
worthwhile, and all the tricky inlining and dispatching made the code
less readable.

Text size: 11015 bytes -> 7467 bytes, and benchmarks show no loss of
performance.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>