kernel/fs: Larger sizes for EntryCache if there is >= 1GB RAM available.

Instead of 8096 maximum entries, now there will be about 130k.
As there are around ~32k files in the Haiku git tree, this has a
serious impact on "git status" performance: in my testing it sped up
from around 0.95s to 0.39s, or less than half with a "hot" cache
(in a VM backed by an NVMe SSD, may be more dramatic on spinning system.)

Compile performance does not seem very much improved, however.

src/system/kernel/fs/EntryCache.cpp

@@ -7,6 +7,7 @@
 #include "EntryCache.h"
 
 #include <new>
+#include <vm/vm.h>
 
 
 static const int32 kEntryNotInArray = -1;
@@ -78,8 +79,15 @@ EntryCache::Init()
 	if (error != B_OK)
 		return error;
 
-	fGenerationCount = 8;
 	int32 entriesSize = 1024;
+	fGenerationCount = 8;
+
+	// TODO: Choose generation size/count more scientifically?
+	// TODO: Add low_resource handler hook?
+	if (vm_available_memory() >= (1024*1024*1024)) {
+		entriesSize = 8096;
+		fGenerationCount = 16;
+	}
 
 	fGenerations = new(std::nothrow) EntryCacheGeneration[fGenerationCount];
 	for (int32 i = 0; i < fGenerationCount; i++) {