nilfs2: optimize rec_len functions

This is a similar change to those in ext2/ext3 codebase (commit
40a063f6 and a4ae3094, respectively).

The addition of 64k block capability in the rec_len_from_disk and
rec_len_to_disk functions added a bit of math overhead which slows
down file create workloads needlessly when the architecture cannot
even support 64k blocks.  This will cut the corner.

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