Leaking path for search_binary_handler

[Tong Zhang]

Kernel Version: 4.18.5

Problem Description:

search_binary_handler() should be called after setting bprm using prepare_binprm(),
and in prepare_binprm(), there’s a LSM hook security_bprm_set_creds(), 
which can make a decision that binfmt cares.

We found a leaking path In fs/binfmt_misc.c:235, that don’t ask LSM’s decision.

- Tong