perf header: Clean up use of perf_env

Always use the perf_env from the feat_fd's perf_header. Cache the
value on entry to a function in `env` and use `env->` consistently in
the code. Ensure the header is initialized for use in
perf_session__do_write_header.

Signed-off-by: Ian Rogers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

Author: captain5050