crypto: x86 - exit fpu context earlier in ECB/CBC macros

Currently the ecb/cbc macros hold fpu context unnecessarily when using
scalar cipher routines (e.g. when handling odd sizes of blocks per walk).

Change the macros to drop fpu context as soon as the fpu is out of use.

No performance impact found (on Intel Haswell).

Signed-off-by: Peter Lafreniere <peter@n8pjl.ca>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>