smalloc: use SMALLOC_BPI instead of SMALLOC_BPB in add_pool()

smalloc: use SMALLOC_BPI instead of SMALLOC_BPB in add_pool()

Change the calculation of free_blocks in add_pool() to use SMALLOC_BPI
instead of SMALLOC_BPB. These two constants are coincidentally the same
on Linux and Windows but SMALLOC_BPI is the correct one to use.
free_blocks is the number of available blocks of size SMALLOC_BPB. It is
the product of the number of unsigned integers in the bitmap
(bitmap_blocks) and the number of bits per unsigned integer
(SMALLOC_BPI).

Signed-off-by: Jens Axboe <axboe@kernel.dk>