ANDROID: GKI: fs: add umount_end() function to struct super_operations

This reduces the ABI diff for the module_layout symbol.

Bug: 148872640
Test: compile test
(cherry picked from commit ee7bdc62)
[willmcvicker: Only cherry-picked the ABI difference]
Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I1e7e110316d90d190cb4fa1dcd18b70d0d6045f5