ANDROID: sdcardfs: Fix case insensitive lookup

The previous case insensitive lookup relied on the
entry being present in the dcache. This instead uses
iterate_dir to find the correct case.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
bug: 35633782
Change-Id: I556f7090773468c1943c89a5e2aa07f746ba49c5