block-backend: use bdrv_getlength instead of blk_getlength

The only difference is that blk_ checks if the block is available,
but this check is already performed above in blk_check_byte_request().

This is in preparation for the graph rdlock, which will be taken
by both the callers of blk_check_byte_request() and blk_getlength().

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230113204212.359076-8-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>