Allow fscrypt_mount_metadata_encrypted to be called more than once

In case of userspace reboot, this function will be called again to
remount userdata into checkpoint mode.

Test: adb shell setprop sys.init.userdata_remount.force_umount_f2fs 1
Test: adb shell /system/bin/vdc checkpoint startCheckpoint 1
Test: adb reboot userspace
Test: adb shell dumpsys activity
Bug: 135984674
Bug: 143970043
Change-Id: I300b1960a6c4e95b13c43d806c4f3a9e4a612ac4