vfs: in iomap seek_{hole,data}, return -ENXIO for negative offsets

In the iomap implementations of SEEK_HOLE and SEEK_DATA, make sure we
return -ENXIO for negative offsets.

Inspired-by: Mateusz S <muttdini@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git a/fs/iomap.c b/fs/iomap.c
index 432eed8..16f5c074 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -610,8 +610,8 @@
 	loff_t length = size - offset;
 	loff_t ret;
 
-	/* Nothing to be found beyond the end of the file. */
-	if (offset >= size)
+	/* Nothing to be found before or beyond the end of the file. */
+	if (offset < 0 || offset >= size)
 		return -ENXIO;
 
 	while (length > 0) {
@@ -656,8 +656,8 @@
 	loff_t length = size - offset;
 	loff_t ret;
 
-	/* Nothing to be found beyond the end of the file. */
-	if (offset >= size)
+	/* Nothing to be found before or beyond the end of the file. */
+	if (offset < 0 || offset >= size)
 		return -ENXIO;
 
 	while (length > 0) {