target/file: Fix 32-bit highmem breakage for SGL -> iovec mapping

This patch changes vectored file I/O to use kmap + kunmap when mapping
incoming SGL memory -> struct iovec in order to properly support 32-bit
highmem configurations.  This is because an extra bounce buffer may be
required when processing scatterlist pages allocated with GFP_KERNEL.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_file.c

@@ -236,14 +236,16 @@ static int fd_do_readv(struct se_cmd *cmd, struct scatterlist *sgl,
 
 	for_each_sg(sgl, sg, sgl_nents, i) {
 		iov[i].iov_len = sg->length;
-		iov[i].iov_base = sg_virt(sg);
+		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset;
 	}
 
 	old_fs = get_fs();
 	set_fs(get_ds());
 	ret = vfs_readv(fd, &iov[0], sgl_nents, &pos);
 	set_fs(old_fs);
 
+	for_each_sg(sgl, sg, sgl_nents, i)
+		kunmap(sg_page(sg));
 	kfree(iov);
 	/*
 	 * Return zeros and GOOD status even if the READ did not return
@@ -288,14 +290,17 @@ static int fd_do_writev(struct se_cmd *cmd, struct scatterlist *sgl,
 
 	for_each_sg(sgl, sg, sgl_nents, i) {
 		iov[i].iov_len = sg->length;
-		iov[i].iov_base = sg_virt(sg);
+		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset;
 	}
 
 	old_fs = get_fs();
 	set_fs(get_ds());
 	ret = vfs_writev(fd, &iov[0], sgl_nents, &pos);
 	set_fs(old_fs);
 
+	for_each_sg(sgl, sg, sgl_nents, i)
+		kunmap(sg_page(sg));
+
 	kfree(iov);
 
 	if (ret < 0 || ret != cmd->data_length) {