crypto: aesni - xts_crypt() return if walk.nbytes is 0

xts_crypt() code doesn't call kernel_fpu_end() after calling
kernel_fpu_begin() if walk.nbytes is 0. The correct behavior should be
not calling kernel_fpu_begin() if walk.nbytes is 0.

Reported-by:  <syzbot+20191dc583eff8602d2d@syzkaller.appspotmail.com>
Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>