xfs: fold xfs_bmbt_init_common into xfs_bmbt_init_cursor

Make the levels initialization in xfs_bmbt_init_cursor conditional
and merge the two helpers.

This requires the fakeroot case to now pass a -1 whichfork directly
into xfs_bmbt_init_cursor, and some special casing for that, but
at least this scheme to deal with the fake btree root is handled and
documented in once place now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: tidy up a multline ternary]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>