seastore: add blocks_per_segment to use paddr_t in RBM

seastore: add blocks_per_segment to use paddr_t in RBM

Seastore exploits paddr_t to represent its address.
However. the term of "segment" in paddr_t generally be used in LFS,
not random block design---"block" is more general term of RBM.

The problem is that seasore already has been tighly coupled with padd_t,
also it does not make sense that there are two differnt address
such as paddr_t and block_paddr_t.

Therefore, to solve this problem, this commit adds blocks_per_segment, which
describes how many blocks a single segment contains to make RBM be compotible
with paddr_t

Signed-off-by: Myoungwon Oh <myoungwon.oh@samsung.com>