ext2: Fix link count corruption under heavy link+rename load

vfs_rename_other() does not lock renamed inode with i_mutex. Thus changing
i_nlink in a non-atomic manner (which happens in ext2_rename()) can corrupt
it as reported and analyzed by Josh.

In fact, there is no good reason to mess with i_nlink of the moved file.
We did it presumably to simulate linking into the new directory and unlinking
from an old one. But the practical effect of this is disputable because fsck
can possibly treat file as being properly linked into both directories without
writing any error which is confusing. So we just stop increment-decrement
games with i_nlink which also fixes the corruption.

CC: stable@kernel.org
CC: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Jan Kara <jack@suse.cz>