ntdll: Avoid one of the calls to getattrlist() when read_directory_getattrlist() checks the case sensitivity of the file system.

Extract a new function, get_dir_case_sensitivity_attr_by_id(), from
get_dir_case_sensitivity_attr().  Call that instead of get_dir_case_sensitivity()
from read_directory_getattrlist().  We get the device and file system IDs from
the same getattrlist() call we're already doing, so that avoids an extra call.