submodule: used correct index in is_staging_gitmodules_ok

submodule: used correct index in is_staging_gitmodules_ok

Commit 883e248b8 (fsmonitor: teach git to optionally utilize a file
system monitor to speed up detecting new or changed files., 2017-09-22)
introduced a call to 'ce_match_stat()' in 'is_staging_gitmodules_ok()'
which implicitly relys on the the global 'the_index' instead of the
passed in 'struct index_state'.  Fix this by changing the call to
'ie_match_stat()' and using the passed in index_state struct.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>