Skip dump_buf_hex() when the log level is low enough

Skip dump_buf_hex() when the log level is low enough

This is a performance fix.

Indeed, dump_buf_hex() is heavy enough to significantly slow down
OpenConnect. So avoid calling dump_buf_hex() when the log level does
not require it.

Before this patch, the whole body of dump_buf_hex() was executed,
even when vpn_progress() wouldn't print anything.

Same for dump_buf(), although the performance hit is less important.

Signed-off-by: Dimitri Papadopoulos <3350651-DimitriPapadopoulos@users.noreply.gitlab.com>