fs_mgr: clean up dm ioctl flags

DM_READONLY_FLAG should only be used when loading tables, and
DM_STATUS_TABLE_FLAG should only be used when querying a table's status. This
patch cleans up how we set flags to reflect when the kernel actually cares
about them.

Bug: 78914864
Test: AVB device still boots

Change-Id: I809d8c2ef2105849ebdd095bbe7f08f15ae63465