ocfs2: deal with wraparounds of i_nlink in ocfs2_rename()

unfortunately, nlink_t may be smaller than 32 bits and ->i_nlink
on ocfs2 can grow up to 0xffffffff; storing it in nlink_t variable
will lose upper bits on such architectures.  Needs to be made u32,
until we get kernel-side nlink_t uniformly 32bit...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>