ovl: fix lseek overflow on 32bit

ovl_lseek() is using ssize_t to return the value from vfs_llseek().  On a
32-bit kernel ssize_t is a 32-bit signed int, which overflows above 2 GB.

Assign the return value of vfs_llseek() to loff_t to fix this.
Reported-by: NBoris Gjenero <boris.gjenero@gmail.com>
Fixes: 9e46b840 ("ovl: support stacked SEEK_HOLE/SEEK_DATA")
Cc: <stable@vger.kernel.org> # v4.19
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -157,7 +157,7 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
 	struct inode *inode = file_inode(file);
 	struct fd real;
 	const struct cred *old_cred;
-	ssize_t ret;
+	loff_t ret;
 
 	/*
 	 * The two special cases below do not need to involve real fs,