[PATCH] check-uapi: preserve failure status for 256 broken headers

check_uapi_files() returns the count of failed headers. Shell exit
statuses are limited to eight bits, so 256 failures become status
zero. The caller then exits successfully despite reporting an ABI
break. Return the documented FAIL_ABI status whenever at least one
header fails, and SUCCESS otherwise.

Signed-off-by: Omer Zalman <redacted>

The exact git-format-patch file is attached because Gmail's web
composer removes tabs from inline diffs.