os/bluestore: Writer, fix find_mutable_blob

os/bluestore: Writer, fix find_mutable_blob

1) Algorithm assumed that blob->blob_start() is aligned to csum size.
It is true for blobs created by write_v2, but write_v1 can generate
blob like: begin = 0x9000, size = 0x6000, csum = 0x2000.
2) Blobs with unused were selected even if those need to be expanded.
This is illegal since we cannot expand unused.

Fixed blob selection algorithm.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>