Revert "fs_mgr_fstab: removing fs_mgr_get_entry_for_mount_point_after()"

This reverts commit c55f1882.

but keeps the change to fs_mgr_get_entry_for_mount_point args:
    fs_mgr_get_entry_for_mount_point(struct fstab* fstab, const char* path);
==> fs_mgr_get_entry_for_mount_point(struct fstab* fstab, const std::string& path);

Change-Id: I19ef4539c5d5be8e1ec83642448814d2c3711c0a