random: Call getrandom before select and emitting a progress callback.

* random/rndlinux.c (_gcry_rndlinux_gather_random): Move the getrandom
call before the select.

A select for getrandom does not make any sense because there is no
file descriptor for getrandom. Thus if getrandom is available we now
select only when we want to read from the blocking /dev/random. In
most cases this avoids all progress callbacks.

• Signed-off-by: Werner Koch <wk@gnupg.org>