os/bluestore: Fix BlueStore::_do_remove

os/bluestore: Fix BlueStore::_do_remove

Fix blobs having the same empty shared blob.

Each blob on creation gets its own unique (empty) SharedBlob object.
ExtentMap::dup() sometimes merges blobs together, so 2 different blobs
get the same SharedBlob object.

Function _do_remove() tries to convert shared blobs into regular ones.
If it succeeds we could get 2 blobs having the same EMPTY SharedBlob object.

The solution is to create detached SharedBlob if necessary.

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