fsstress: avoid splice_f generating too large sparse file

Thanks to Darrick J. Wong find this issue! Current splice_f generates
file offset as below:

  lr = ((int64_t)random() << 32) + random();
  off2 = (off64_t)(lr % maxfsize);

It generates a pseudorandom 64-bit candidate offset for the
destination file where we'll land the splice data, and then caps the
offset at maxfsize (which is 2^63- 1 on x64), which effectively means
that the data will appear at a very high file offset which creates
large (sparse) files very quickly.

That's not what we want, and some case likes shared/009 will take
forever to run md5sum on lots of huge files.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

ltp/fsstress.c

@@ -2862,10 +2862,11 @@ splice_f(int opno, long r)
 
 	/*
 	 * splice can overlap write, so the offset of the target file can be
-	 * any number (< maxfsize)
+	 * any number. But to avoid too large offset, add a clamp of 1024 blocks
+	 * past the current dest file EOF
 	 */
 	lr = ((int64_t)random() << 32) + random();
-	off2 = (off64_t)(lr % maxfsize);
+	off2 = (off64_t)(lr % MIN(stat2.st_size + (1024ULL * stat2.st_blksize), MAXFSIZE));
 
 	/*
 	 * Due to len, off1 and off2 will be changed later, so record the