xfs: exchange-range for repairs is no longer dynamic

The atomic file exchange-range functionality is now a permanent
filesystem feature instead of a dynamic log-incompat feature.  It cannot
be turned on at runtime, so we no longer need the XCHK_FSGATES flags and
whatnot that supported it.  Remove the flag and the enable function, and
move the xfs_has_exchange_range checks to the start of the repair
functions.

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