xfstests: randholes: Fix two bugs

This patch fixes two bugs in the "randholes" test program.

First, it is possible for findblock() to return -1 if the random block number it picks is at or above the highest in-range block that's already been selected. But this case isn't checked and the value is blindly used thereafter as if it were valid. Just exit if this ever occurs.

Second, when the "alloconly" option is is set, blocks are preallocated in the target file rather than actually writing them. But unlike when the blocks are written and subsequently read, the preallocated blocks are *not* offset by the fileoffset parameter.

I'm pretty sure nobody every noticed this because the program itself doesn't do any verification when blocks are only preallocated. But it's an inconsistency and I think it ought to be fixed.

---
 src/randholes.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: b/src/randholes.c
===================================================================
@@ -249,11 +249,16 @@ writeblks(char *fname, int fd)
 			fflush(stdout);
 		}
 		block = findblock();
-		offset = (__uint64_t)block * blocksize;
+		if (block < 0) {
+		    perror("findblock");
+		    exit(1);
+		}
+
+		offset = (__uint64_t) block * blocksize;
 		if (alloconly) {
                         if (test) continue;
                         
-			fl.l_start = offset;
+			fl.l_start = fileoffset + offset;
 			fl.l_len = blocksize;
 			fl.l_whence = 0;
 

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