mount-encrypted: fix some minor security TODOs
mount-encrypted: fix some minor security TODOs

Force mode of created key file to 0600, and make sure there is enough
room in the decryption buffer for any possible change to the decryption
algo.

BUG=None
TEST=alex build, manual testing

Change-Id: I89dceec22683ff66b5e1f61a63f14a1db1c4e2ee
Signed-off-by: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/28892
Reviewed-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>

ionave/seismograph,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,ionave/seismograph,marineam/seismograph,acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,ccaapton/vboot_reference,geekboxzone/mmallow_external_vboot_reference,marineam/seismograph,acorn-marvell/vboot_reference,marineam/seismograph,coreboot/vboot,acorn-marvell/vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,marineam/seismograph,ccaapton/vboot_reference,ionave/seismograph