fs_mgr_verity: fix androidboot.veritymode=enforcing doesn't work

Should set verity mode to VERITY_MODE_DEFAULT when
androidboot.veritymode=enforcing.

Bug: 38103331
Test: `adb shell getprop partition.system.verified` returns 2 when veritymode=enforcing
Test: `adb shell getprop partition.vendor.verified` returns 2 when veritymode=enforcing

(cherry picked from commit 77cbfd63)

Merged-Id: I2f34eb6509f91989ce726e540cf2c0d353347ede
Change-Id: I2f34eb6509f91989ce726e540cf2c0d353347ede