mac: add readahead control to the posix_fadvise() shim

- Add support for POSIX_FADV_NORMAL in the posix_fadvise() shim by just
  ignoring it
- Add support for POSIX_FADV_SEQUENTIAL/POSIX_FADV_RANDOM by mapping
  them to enable/disable of readahead via fcntl(..., F_RDAHEAD, ...).
  Because macOS only lets you control readahead at the descriptor level
  the offset and len values passed will be  ignored and range control is
  not done.

The impact of being able to tune readahead can be seen below:

./fio --filename=fio.tmp --stonewall --size=128M --filename=fio.tmp --bs=4k \
  --name=precache --rw=read --bs=128k \
  --name=cached --rw=read \
  --name=uncached-readahead --rw=read --invalidate=1 \
  --name=uncached-no-readahead --rw=read --invalidate=1 --fadvise=random

[...]
cached: (groupid=1, jobs=1): err= 0: pid=32600: Sat May 17 18:49:22 2025
  read: IOPS=349k, BW=1362MiB/s (1428MB/s)(128MiB/94msec)
--
uncached-readahead: (groupid=2, jobs=1): err= 0: pid=32601: Sat May 17 18:49:22 2025
  read: IOPS=315k, BW=1231MiB/s (1291MB/s)(128MiB/104msec)
--
uncached-no-readahead: (groupid=3, jobs=1): err= 0: pid=32602: Sat May 17 18:49:22 2025
  read: IOPS=40.8k, BW=159MiB/s (167MB/s)(128MiB/803msec)

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>

Author: sitsofe

os/mac/posix.c

@@ -1,4 +1,6 @@
 #include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
 #include <sys/mman.h>
@@ -58,15 +60,30 @@ static int discard_pages(int fd, off_t offset, off_t len)
 	return 0;
 }
 
+static inline int set_readhead(int fd, bool enabled) {
+	int ret;
+
+	ret = fcntl(fd, F_RDAHEAD, enabled ? 1 : 0);
+	if (ret == -1) {
+		ret = errno;
+	}
+
+	return ret;
+}
+
 int posix_fadvise(int fd, off_t offset, off_t len, int advice)
 {
 	int ret;
 
 	switch(advice) {
 	case POSIX_FADV_NORMAL:
+		ret = 0;
+		break;
 	case POSIX_FADV_RANDOM:
+		ret = set_readhead(fd, false);
+		break;
 	case POSIX_FADV_SEQUENTIAL:
-		ret = 0;
+		ret = set_readhead(fd, true);
 		break;
 	case POSIX_FADV_DONTNEED:
 		ret = discard_pages(fd, offset, len);