nilfs2: get maximum number of child nodes from bmap object

The patch "reduce repetitive calculation of max number of child nodes"
gathered up the calculation of maximum number of child nodes into
nilfs_btree_nchildren_per_block() function.  This makes the function
get resultant value from a private variable in bmap object instead of
calculating it for each call.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>