nilfs2: fix buffer head leak in nilfs_btnode_submit_block

nilfs_btnode_submit_block() refers to buffer head just before
returning from the function, but it releases the buffer head earlier
than that if nilfs_dat_translate() gets an error.

This has potential for oops in the erroneous case.  This fixes the
issue.

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