init: rework fixup_sys_perms to use perm_path_matches

fixup_sys_perms does the same matching as the function perm_path_matches
other than the leading '/sys'. Adjust the path and use the common
matching function. Replace the fixed length C strings with std::string.

Change-Id: Ic09d91aa4ac1f1096018617537d782b82ee9ef63
Signed-off-by: Rob Herring <robh@kernel.org>