os/bluestore/BlueStore: Don't leak trim overlay data before write.

os/bluestore/BlueStore: Don't leak trim overlay data before write.

Suppose: bluestore_overlay_max_length=bluestore_min_alloc_size;
bluestore_overlay_max = 2;
For the following ops:
write(off=0, len=4096) --->write into overlay
write(off=4096, len=4096)-->write into overlay
write(off=0, len=bluestore_min_alloc_size)-->because overlay_map.size()
>=2, it allocate a extent.

It should trim overlay data(0,4096) &(4096, 4096),and then write(0,
bluestore_min_alloc_size).
But the original code don't trim overlay data.
This make the later read data is orignal data rather that new data.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>