null_encrypt() needs to copy the data in case src and dst are disjunct,
null_compress() needs to copy the data in any case as far as I can tell.  I
joined compress/decompress and encrypt/decrypt to avoid duplicating code.

Without this patch ESP null_enc packets look like this:

IP (tos 0x0, ttl  64, id 23130, offset 0, flags [DF], length: 128)
10.0.0.1 > 10.0.0.2: ESP(spi=0x0f9ca149,seq=0x4)
	0x0000:  4500 0080 5a5a 4000 4032 cbef 0a00 0001  E...ZZ@.@2......
	0x0010:  0a00 0002 0f9c a149 0000 0004 0000 0000  .......I........
	0x0020:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0030:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0040:  0000 0000 0000 0000 0000 0000 0000 0000  ................
	0x0050:  0000                                     ..

IP (tos 0x0, ttl  64, id 256, offset 0, flags [DF], length: 128)
10.0.0.2 > 10.0.0.1: ESP(spi=0x0e4f7b51,seq=0x2)
	0x0000:  4500 0080 0100 4000 4032 254a 0a00 0002  E.....@.@2%J....
	0x0010:  0a00 0001 0e4f 7b51 0000 0002 a8a8 a8a8  .....O{Q........
	0x0020:  a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8  ................
	0x0030:  a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8  ................
	0x0040:  a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8 a8a8  ................
	0x0050:  a8a8                                     ..
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>