X don't overalign end by 1 blksize if end is already aligned

Else:

	wcfs: 2018/10/10 17:52:15 < 40: i7.READ {Fh 0 [4063232 +131072)  L 0 RDONLY,0x8000}
	wcfs: 2018/10/10 17:52:15 > 39:     OK,  131072B data
	wcfs: 2018/10/10 17:52:15 > 40:     OK,  131072B data
	wcfs: 2018/10/10 17:52:15 < 41: i7.GETATTR {Fh 0}
	wcfs: 2018/10/10 17:52:15 Response: INODE_NOTIFY_STORE_CACHE: OK
	wcfs: 2018/10/10 17:52:15 > 41:     OK, {tA=1s {M0100444 SZ=4194304 L=1 1000:1000 B0*0 i0:7 A 0.000000 M 1539183135.261177 C 1539183135.261177}}

	# XXX vvv why we store 2M after read @4M even though read gives len=0 ?
	wcfs: 2018/10/10 17:52:15 > 0:     NOTIFY_STORE_CACHE, {i7 [4194304 +2097152)} 2097152B data
	wcfs: 2018/10/10 17:52:15 < 42: i7.READ {Fh 0 [4194304 +4096)  L 0 RDONLY,0x8000}
	wcfs: 2018/10/10 17:52:15 > 42:     OK,

	wcfs: 2018/10/10 17:52:15 < 43: i7.GETATTR {Fh 0}
	wcfs: 2018/10/10 17:52:15 > 43:     OK, {tA=1s {M0100444 SZ=4194304 L=1 1000:1000 B0*0 i0:7 A 0.000000 M 1539183135.261177 C 1539183135.261177}}
	wcfs: 2018/10/10 17:52:15 Response: INODE_NOTIFY_STORE_CACHE: OK
	wcfs: 2018/10/10 17:52:15 < 44: i7.READ {Fh 0 [4198400 +4096)  L 0 RDONLY,0x8000}
	wcfs: 2018/10/10 17:52:15 > 44:     OK,

        data = readfile(fpath + "/head/data")
>       assert len(data) == fsize
E       AssertionError: assert 4198400 == 4194304

wcfs/wcfs.go

@@ -518,7 +518,10 @@ func (bfdata *BigFileData) Read(_ nodefs.File, dest []byte, off int64, fctx *fus
 	// widen read request to be aligned with blksize granularity
 	// (we can load only whole ZBlk* blocks)
 	aoff := off - (off % zbf.blksize)
-	aend := end + (zbf.blksize - (end % zbf.blksize))
+	aend := end
+	if re := end % zbf.blksize; re != 0 {
+		aend += zbf.blksize - re
+	}
 	dest = make([]byte, aend - aoff) // ~> [off:aend] in file
 
 	// XXX better ctx = transaction.PutIntoContext(ctx, txn)