os/bluestore: detect unnecessary zeros in _do_write_small()

os/bluestore: detect unnecessary zeros in _do_write_small()

Bluestore's `_do_write()` method handles writing data from bufferlists. Currently, it writes data from bufferlists without checking for unnecessary zeros. The lack zero detection may negatively impact performance.

In _do_write_small(), we check if a bufferlist is made up of zeros and avoid writing it if so.
Two new counters, `l_bluestore_write_small_skipped` and l_bluestore_write_small_skipped_bytes`, have been introduced to help us count how many zero blocks and bytes from _do_write_small() have been skipped.

Signed-off-by: Laura Flores <lflores@redhat.com>