libxfs: port the bumplink function from the kernel

libxfs: port the bumplink function from the kernel

Port the xfs_bumplink function from the kernel and use it to replace raw
calls to inc_nlink.  The next patch will need this common function to
prevent integer overflows in the link count.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>