ralloc: don't memset ralloc_header, clear it manually

ralloc: don't memset ralloc_header, clear it manually

time GALLIUM_NOOP=1 ./run shaders/private/alien_isolation/ >/dev/null

Before (2 takes):

real    0m8.734s    0m8.773s
user    0m34.232s   0m34.348s
sys     0m0.084s    0m0.056s

After (2 takes):

real    0m8.448s    0m8.463s
user    0m33.104s   0m33.160s
sys     0m0.088s    0m0.076s

Average change in "real" time spent: -3.4%

calloc should only do 2 things compared to malloc:
- check for overflow of "n * size"
- call memset
I'm not sure if that explains the difference.

v2: clear "parent" and "next" in the caller of add_child.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net> (v1)
Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com> (v1)
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v1)