Linux 5.16 compat: Added mapping for iov_iter_fault_in_readable

Linux decided to rename this for some reason. At some point, we
should probably invert this mapping, but for now...

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

config/kernel-add-disk.m4

@@ -13,7 +13,6 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_ADD_DISK], [
 	])
 
 ])
-
 AC_DEFUN([ZFS_AC_KERNEL_ADD_DISK], [
 	AC_MSG_CHECKING([whether add_disk() returns int])
 	ZFS_LINUX_TEST_RESULT([add_disk_ret],

config/kernel-vfs-iov_iter.m4

@@ -41,6 +41,17 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_VFS_IOV_ITER], [
 		error = iov_iter_fault_in_readable(&iter, size);
 	])
 
+	ZFS_LINUX_TEST_SRC([fault_in_iov_iter_readable], [
+		#include <linux/fs.h>
+		#include <linux/uio.h>
+	],[
+		struct iov_iter iter = { 0 };
+		size_t size = 512;
+		int error __attribute__ ((unused));
+
+		error = fault_in_iov_iter_readable(&iter, size);
+	])
+
 	ZFS_LINUX_TEST_SRC([iov_iter_count], [
 		#include <linux/fs.h>
 		#include <linux/uio.h>
@@ -123,8 +134,15 @@ AC_DEFUN([ZFS_AC_KERNEL_VFS_IOV_ITER], [
 		AC_DEFINE(HAVE_IOV_ITER_FAULT_IN_READABLE, 1,
 		    [iov_iter_fault_in_readable() is available])
 	],[
-		AC_MSG_RESULT(no)
-		enable_vfs_iov_iter="no"
+		AC_MSG_CHECKING([whether fault_in_iov_iter_readable() is available])
+		ZFS_LINUX_TEST_RESULT([fault_in_iov_iter_readable], [
+			AC_MSG_RESULT(yes)
+			AC_DEFINE(HAVE_FAULT_IN_IOV_ITER_READABLE, 1,
+			    [fault_in_iov_iter_readable() is available])
+		],[
+			AC_MSG_RESULT(no)
+			enable_vfs_iov_iter="no"
+		])
 	])
 
 	AC_MSG_CHECKING([whether iov_iter_count() is available])

include/os/linux/spl/sys/uio.h

@@ -34,6 +34,10 @@
 #include <asm/uaccess.h>
 #include <sys/types.h>
 
+#if defined(HAVE_VFS_IOV_ITER) && defined(HAVE_FAULT_IN_IOV_ITER_READABLE)
+#define	iov_iter_fault_in_readable(a, b)	fault_in_iov_iter_readable(a, b)
+#endif
+
 typedef struct iovec iovec_t;
 
 typedef enum zfs_uio_rw {