Don't bother with redoing rw_verify_area() from default_file_splice_from()

default_file_splice_from() ends up calling vfs_write() (via very convoluted
callchain).  It's an overkill, since we already have done rw_verify_area()
in the caller by the time we call vfs_write() we are under set_fs(KERNEL_DS),
so access_ok() is also pointless.  Add a new helper (__kernel_write()),
use it instead of kernel_write() in there.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/internal.h

@@ -125,3 +125,8 @@ extern int invalidate_inodes(struct super_block *, bool);
  * dcache.c
  */
 extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
+
+/*
+ * read_write.c
+ */
+extern ssize_t __kernel_write(struct file *, const char *, size_t, loff_t *);

fs/read_write.c

@@ -17,6 +17,7 @@
 #include <linux/splice.h>
 #include <linux/compat.h>
 #include "read_write.h"
+#include "internal.h"
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -417,6 +418,30 @@ ssize_t do_sync_write(struct file *filp, const char __user *buf, size_t len, lof
 
 EXPORT_SYMBOL(do_sync_write);
 
+ssize_t __kernel_write(struct file *file, const char *buf, size_t count, loff_t *pos)
+{
+	mm_segment_t old_fs;
+	const char __user *p;
+	ssize_t ret;
+
+	old_fs = get_fs();
+	set_fs(get_ds());
+	p = (__force const char __user *)buf;
+	if (count > MAX_RW_COUNT)
+		count =  MAX_RW_COUNT;
+	if (file->f_op->write)
+		ret = file->f_op->write(file, p, count, pos);
+	else
+		ret = do_sync_write(file, p, count, pos);
+	set_fs(old_fs);
+	if (ret > 0) {
+		fsnotify_modify(file);
+		add_wchar(current, ret);
+	}
+	inc_syscw(current);
+	return ret;
+}
+
 ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
 {
 	ssize_t ret;

fs/splice.c

@@ -31,6 +31,7 @@
 #include <linux/security.h>
 #include <linux/gfp.h>
 #include <linux/socket.h>
+#include "internal.h"
 
 /*
  * Attempt to steal a page from a pipe buffer. This should perhaps go into
@@ -1048,9 +1049,10 @@ static int write_pipe_buf(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 {
 	int ret;
 	void *data;
+	loff_t tmp = sd->pos;
 
 	data = buf->ops->map(pipe, buf, 0);
-	ret = kernel_write(sd->u.file, data + buf->offset, sd->len, sd->pos);
+	ret = __kernel_write(sd->u.file, data + buf->offset, sd->len, &tmp);
 	buf->ops->unmap(pipe, buf, data);
 
 	return ret;