xfstests: btrfs/004, fix filefrag filter for files with 1 block only

If the file consists of a single block, then filefrag mentions '1 block of ...', and the filter expected 'blocks of ...'.

Example:

$ echo qwerty > foobar
$ filefrag -v foobar
Filesystem type is: ef53
File size of foobar is 7 (1 block of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:          0..         0:      1:             unknown,delalloc,eof
foobar: 1 extent found

---
 tests/btrfs/004 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 211d8bc..670e1c2 100755
@@ -58,7 +58,7 @@ _require_command "/usr/sbin/filefrag"
 rm -f $seqres.full
 
 FILEFRAG_FILTER='
-	if (/blocks of (\d+) bytes/) {
+	if (/blocks? of (\d+) bytes/) {
 		$blocksize = $1;
 		next
 	}

1.9.1