generic/304: only dedupe the last 64k of the single byte file

From: Darrick J. Wong <[email protected]>

Commit 1ddae54555b62 ("common/rc: add missing 'local' keywords") exposed a long-hidden bug in generic/304 -- previously we'd set len to 8EiB, but _pwrite_byte reset it to 1 because the helper clumsily polluted the caller's variable namespace. Now that's fixed, but we send an 8EiB dedupe request to the kernel, which on XFS locks up the kernel while doing this. The point of this test is to demonstrate that one cannot dedupe the last byte of a (2^63-1) byte file (that's the way the interface has behaved historically), so start at 64k below that instead of offset zero.

---
 tests/generic/304 |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 6c36a68..ff8ddf8 100755
@@ -63,7 +63,7 @@ _pwrite_byte 0x61 $bigoff 1 $testdir/file3 >> $seqres.full
 _pwrite_byte 0x61 1048575 1 $testdir/file2 >> $seqres.full
 
 echo "Dedupe large single byte file"
-_dedupe_range $testdir/file1 0 $testdir/file3 0 $len \
+_dedupe_range $testdir/file1 $bigoff_64k $testdir/file3 $bigoff_64k 65536 \
 	2>&1 >> $seqres.full | _filter_dedupe_error
 
 echo "Dedupe large empty file"