nilfs2: add free entries count only if clear bit operation succeeded

Three functions of the current persistent object allocator,
nilfs_palloc_commit_free_entry, nilfs_palloc_abort_alloc_entry, and
nilfs_palloc_freev functions unconditionally add a counter after doing
clear bit operation on a bitmap block.

If the clear bit operation overlapped, the counter will not add up.
This fixes the issue by making the counter operations conditional.

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