fs_mgr.cpp: fix the problem that the casefold feature not enabled for the data partition

The value of entry.mount_point for data partition is "/data"

Fixes: 5ba5b90c ("fs_mgr: try tune2fs for casefolding on /data only")

Test: got "Can't mount with encoding and encryption" problem reported
      by the db845c build with the default 5.4.38 prebuilt kernel

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: I226a2275f5f2ee18503c5a3863ef5a1d2c2ed7be