box: fix unexpected error on granting privileges to admin

We use LuaJIT 'bit' module for bitwise operations. Due to platform
interoperability it truncates arguments to 32bit and returns signed
result. Thus on granting rights using bit.bor to admin user which
have 0xffffffff rights (from bootstrap snapshot) we get -1 as a result.
This leads to type check error given in issue later in execution.

Closes #7226

NO_DOC=minor bugfix