xfs: cleanup picking the start extent hint in xfs_bmap_rtalloc

Clean up the logical in xfs_bmap_rtalloc that tries to find a rtextent
to start the search from by using a separate variable for the hint, not
calling xfs_bmap_adjacent when we want to ignore the locality and avoid
an extra roundtrip converting between block numbers and RT extent
numbers.

As a side-effect this doesn't pointlessly call xfs_rtpick_extent and
increment the start rtextent hint if we are going to ignore the result
anyway.

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