xfs: make full use of xfs_btree_stage_ifakeroot in xfs_bmbt_stage_cursor

xfs: make full use of xfs_btree_stage_ifakeroot in xfs_bmbt_stage_cursor

Source kernel commit: 579d7022d1afea8f4475d1750224ec0b652febee

Remove the duplicate cur->bc_nlevels assignment in xfs_bmbt_stage_cursor,
and move the cur->bc_ino.forksize assignment into
xfs_btree_stage_ifakeroot as it is part of setting up the fake btree
root.

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>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>