dm persistent data: fix return type of shadow_root()

shadow_root() truncates 64-bit dm_block_t into 32-bit int.  This is
not an issue in practice, since dm metadata as of v5.11 can only hold at
most 4161600 blocks (255 index entries * ~16k metadata blocks).

Nevertheless, this can confuse users debugging some specific data
corruption scenarios.  Also, DM_SM_METADATA_MAX_BLOCKS may be bumped in
the future, or persistent-data may find its use in other places.

Therefore, switch the return type of shadow_root from int to dm_block_t.

Signed-off-by: Jinoh Kang <jinoh.kang.kr@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>