os/bluestore: Make write_v2 not need to use compress_extents

os/bluestore: Make write_v2 not need to use compress_extents

Remove need to call compress_extents.
The only place that could produce compressable extents are
_try_reuse_allocated_[l/r] functions.

Create _place_extent_in_blob() function that adds mapping to blob.
If possible, it expands the extent; if not, creates the new one.
The function is used whenever we need new mapping to an existing Blob.

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