nilfs2: get rid of nilfs_btree uses

This replaces all uses of nilfs_btree struct in implementation of
btree mapping with nilfs_bmap struct.

Name of local variable "btree" is kept not to bloat amount of change.
And, a part of local variables "bmap" is renamed to "btree" to uniform
naming rule.

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