Combine iter/update/trigger/str_hash flags into a single enum, and
x-macroize them for a to_text() function later.

These flags are all for a specific iter/key/update context, so it makes
sense to group them together - iter/update/trigger flags were already
given distinct bits, this cleans up and unifies that handling.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>