xfs: add a xfs_btree_init_ptr_from_cur

Inode-rooted btrees don't need to initialize the root pointer in the
->init_ptr_from_cur method as the root is found by the
xfs_btree_get_iroot method later.  Make ->init_ptr_from_cur option
for inode rooted btrees by providing a helper that does the right
thing for the given btree type and also documents the semantics.

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