Rename geom_grow -> exp_grow.

This was promised in the review of the introduction of geom_grow, but would have
been painful to do there because of the series that introduced it.  Now that
those are comitted, renaming is easier.

src/exp_grow.c

@@ -0,0 +1,8 @@
+#include "jemalloc/internal/jemalloc_preamble.h"
+#include "jemalloc/internal/jemalloc_internal_includes.h"
+
+void
+exp_grow_init(exp_grow_t *exp_grow) {
+	exp_grow->next = sz_psz2ind(HUGEPAGE);
+	exp_grow->limit = sz_psz2ind(SC_LARGE_MAXCLASS);
+}