xfs: fix fsmap cursor handling [v2]

This patchset addresses an integer overflow bug that Dave Chinner found
in how fsmap handles figuring out where in the record set we left off
when userspace calls back after the first call filled up all the
designated record space.

v2: add RVB tags

This has been lightly tested with fstests.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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