crypto: tcrypt - fix uninit sg entries in test_acipher_speed

crypto: tcrypt - fix uninit sg entries in test_acipher_speed

Commit 5be4d4c94b1f ("crypto: replace scatterwalk_sg_next with sg_next")
did not consider the fact that scatterwalk_sg_next() was looking at
sg entry length, while sg_next() looks at the "chained" sg bit.

This should have no effect in theory. However in practice, there are
cases where the sg table is initialized to a number of entries and
some of them are not properly configured. While scatterwalk_sg_next()
would have returned NULL (since sg length = 0 and sg page_link = 0),
sg_next() happily returns the next unconfigured sg entry.

insmod tcrypt.ko mode=500 sec=1

testing speed of async cbc(aes) (cbc-aes-talitos) encryption
test 0 (128 bit key, 16 byte blocks):
Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xc00d79e4
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=8 P1022 DS
Modules linked in: tcrypt(+) talitos
CPU: 0 PID: 2670 Comm: insmod Not tainted 4.0.0-rc1-QorIQ-SDK-V1.6+g904f1ca82209 #1
task: e8de3200 ti: e70bc000 task.ti: e70bc000
NIP: c00d79e4 LR: f92d223c CTR: c00d79c8
REGS: e70bda00 TRAP: 0300   Not tainted  (4.0.0-rc1-QorIQ-SDK-V1.6+g904f1ca82209)
MSR: 00029000 <CE,EE,ME>  CR: 84428f22  XER: 00000000
DEAR: 00000000 ESR: 00000000
GPR00: f92d223c e70bdab0 e8de3200 00000000 e70bdbb8 00000001 00000000 00000000
GPR08: 00000000 00000000 c08b0380 27282010 c00d79c8 1003a634 00000000 e70bdf1c
GPR16: e70bdef0 00000020 00000000 c08c0000 00000010 00000000 e70bdbb8 00000010
GPR24: e976d3a8 00000010 00000000 e70bdbd8 e8961010 00000001 c086e560 00000000
NIP [c00d79e4] page_address+0x1c/0x110
LR [f92d223c] talitos_map_sg+0x130/0x184 [talitos]
Call Trace:
[e70bdab0] [00000010] 0x10 (unreliable)
[e70bdad0] [f92d223c] talitos_map_sg+0x130/0x184 [talitos]
[e70bdb00] [f92d30d8] common_nonsnoop.constprop.13+0xc0/0x304 [talitos]
[e70bdb30] [f933fd90] test_acipher_speed+0x434/0x7dc [tcrypt]
[e70bdcc0] [f934318c] do_test+0x2478/0x306c [tcrypt]
[e70bdd80] [f11fe058] tcrypt_mod_init+0x58/0x100 [tcrypt]
[e70bdda0] [c0002354] do_one_initcall+0x90/0x1f4
[e70bde10] [c061fe00] do_init_module+0x60/0x1ac
[e70bde30] [c00a79f0] load_module+0x185c/0x1f88
[e70bdee0] [c00a82b0] SyS_finit_module+0x7c/0x98
[e70bdf40] [c000e8b0] ret_from_syscall+0x0/0x3c

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>