contentenc: reserve one extra block in pool plaintext buffers

File holes and -fsck can cause unaligned read accesses, which means
we have to decrypt one extra plaintext block.

xfstests generic/083 manage to crash -fsck like this:

generic/083	2018/07/14 15:25:21 wrong len=266240, want=131072
panic: wrong len=266240, want=131072

goroutine 1 [running]:
log.Panicf(0x67fc00, 0x15, 0xc4204fec90, 0x2, 0x2)
	/usr/local/go/src/log/log.go:333 +0xda
github.com/rfjakob/gocryptfs/internal/contentenc.(*bPool).Put(0xc4200d4800, 0xc4202f2000, 0x21000, 0x41000)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/internal/contentenc/bpool.go:27 +0x15d
github.com/rfjakob/gocryptfs/internal/fusefrontend.(*File).doRead(0xc4200b4500, 0xc42019e000, 0x0, 0x20000, 0x28400, 0x20000, 0xc42019e000, 0xc4204ff008, 0x435164, 0xc420000180)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/internal/fusefrontend/file.go:227 +0xba9
github.com/rfjakob/gocryptfs/internal/fusefrontend.(*File).Read(0xc4200b4500, 0xc42019e000, 0x20000, 0x20000, 0x28400, 0x0, 0x0, 0x0)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/internal/fusefrontend/file.go:246 +0x23e
main.(*fsckObj).file(0xc420069320, 0xc42001a630, 0x21)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/fsck.go:126 +0x21f
main.(*fsckObj).dir(0xc420069320, 0xc420014dc0, 0x1d)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/fsck.go:76 +0x387
main.(*fsckObj).dir(0xc420069320, 0xc42021dae0, 0x19)
	/home/jakob/go/src/github.com/rfjakob/gocryptfs/fsck.go:74 +0x347

0 files changed, 0 insertions, 0 deletions