xfs: teach the tempfile to support atomic extent swapping

Create some new routines to exchange the contents of a temporary file
created to stage a repair with another ondisk file.  This will be used
by the realtime summary repair function to commit atomically the new
rtsummary data, which will be staged in the tempfile.

The rest of XFS coordinates access to the realtime metadata inodes
solely through the ILOCK.  For repair to hold its exclusive access to
the realtime summary file, it has to allocate a single large transaction
and roll it repeatedly throughout the repair while holding the ILOCK.
In turn, this means that for now there's only a partial swapext
implementation for the temporary file, because we can only work within
an existing transaction.  Hence the only tempswap functions needed here
are to estimate the resource requirements of swapext between, reserve
more space/quota to an existing transaction, and kick off the actual
swap.  The rest will be added in a later patch in preparation for
repairing xattrs and directories.

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

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