nilfs2: fix kernel oops in error case of nilfs_ioctl_move_blocks

This fixes a kernel oops reported by Markus Trippelsdorf in the email
titled "[NILFS users] kernel Oops while running nilfs_cleanerd".

The oops was caused by a bug of error path in
nilfs_ioctl_move_blocks() function, which was inlined in
nilfs_ioctl_clean_segments().

nilfs_ioctl_move_blocks checks duplication of blocks which will be
moved in garbage collection.  But, the check should have be done
within nilfs_ioctl_move_inode_block() to prevent list corruption among
buffers storing the target blocks.

To fix the kernel oops, this moves forward the duplication check
before the list insertion.

I also tested this for stable trees [2.6.30, 2.6.31].

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: stable <stable@kernel.org>