Add support for FALLOC_FL_ZERO_RANGE

For us, I think it's always just FALLOC_FL_PUNCH_HOLE with a fake
mustache on.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Coleman Kane <ckane@colemankane.org>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes openzfs#12975

config/kernel-fallocate.m4

@@ -3,6 +3,10 @@ dnl # Linux 2.6.38 - 3.x API
 dnl # The fallocate callback was moved from the inode_operations
 dnl # structure to the file_operations structure.
 dnl #
+dnl #
+dnl # Linux 3.15+
+dnl # fallocate learned a new flag, FALLOC_FL_ZERO_RANGE
+dnl #
 AC_DEFUN([ZFS_AC_KERNEL_SRC_FALLOCATE], [
 	ZFS_LINUX_TEST_SRC([file_fallocate], [
 		#include <linux/fs.h>
@@ -15,12 +19,25 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_FALLOCATE], [
 			.fallocate = test_fallocate,
 		};
 	], [])
+	ZFS_LINUX_TEST_SRC([falloc_fl_zero_range], [
+		#include <linux/falloc.h>
+	],[
+		int flags __attribute__ ((unused));
+		flags = FALLOC_FL_ZERO_RANGE;
+	])
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_FALLOCATE], [
 	AC_MSG_CHECKING([whether fops->fallocate() exists])
 	ZFS_LINUX_TEST_RESULT([file_fallocate], [
 		AC_MSG_RESULT(yes)
+		AC_MSG_CHECKING([whether FALLOC_FL_ZERO_RANGE exists])
+		ZFS_LINUX_TEST_RESULT([falloc_fl_zero_range], [
+			AC_MSG_RESULT(yes)
+			AC_DEFINE(HAVE_FALLOC_FL_ZERO_RANGE, 1, [FALLOC_FL_ZERO_RANGE is defined])
+		],[
+			AC_MSG_RESULT(no)
+		])
 	],[
 		ZFS_LINUX_TEST_ERROR([file_fallocate])
 	])

module/os/linux/zfs/zpl_file.c

@@ -739,7 +739,12 @@ zpl_fallocate_common(struct inode *ip, int mode, loff_t offset, loff_t len)
 	fstrans_cookie_t cookie;
 	int error = 0;
 
-	if ((mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE)) != 0)
+	int test_mode = FALLOC_FL_PUNCH_HOLE;
+#ifdef HAVE_FALLOC_FL_ZERO_RANGE
+	test_mode |= FALLOC_FL_ZERO_RANGE;
+#endif
+
+	if ((mode & ~(FALLOC_FL_KEEP_SIZE | test_mode)) != 0)
 		return (-EOPNOTSUPP);
 
 	if (offset < 0 || len <= 0)
@@ -750,7 +755,7 @@ zpl_fallocate_common(struct inode *ip, int mode, loff_t offset, loff_t len)
 
 	crhold(cr);
 	cookie = spl_fstrans_mark();
-	if (mode & FALLOC_FL_PUNCH_HOLE) {
+	if (mode & (test_mode)) {
 		flock64_t bf;
 
 		if (offset > olen)