xfs: dont treat rt extents beyond EOF as eofblocks to be cleared

On a system with a realtime volume and a 28k realtime extent,
generic/491 fails because the test opens a file on a frozen filesystem
and closing it causes xfs_release -> xfs_can_free_eofblocks to
mistakenly think that the the blocks of the realtime extent beyond EOF
are posteof blocks to be freed.  Realtime extents cannot be partially
unmapped, so this is pointless.  Worse yet, this triggers posteof
cleanup, which stalls on a transaction allocation, which is why the test
fails.

Teach the predicate to account for realtime extents properly.

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