MINOR: activity: make the memory profiling hash size configurable at build time

The MEMPROF_HASH_BITS variable was set to 10 without a possibility to
change it (beyond patching the code). After seeing a few reports already
with "other" being listed and a list with close to 1024 entries, it looks
like it's about time to either increase the hash size, or at least make
it configurable for special cases. As a reminder, in order to remain
fast, the algorithm searches no more than 16 places after the hash, so
when a table is almost full, searches are long and new places are rare.

The present patch just makes it possible to redefine it by passing
"-DMEMPROF_HASH_BITS=11" or "-DMEMPROF_HASH_BITS=12" in CFLAGS, and
moves the definition to defaults.h to make it easier to find. Such
values should be way sufficient for the vast majority of use cases.
Maybe in the future we'd change the default. At least this version
should be backported to ease rebuilds, say, till 2.8 or so.

(cherry picked from commit 290659ffd3a2eead918adc387e8842c59fbff2e7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit dd42e4233d2c6cc328e61ac0bf8f3375de1667e9)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit b5edee5d7faebc124d20dc8495b78a7f07f1bbd6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
2 files changed