xfstests: rename RETAIN_AG_BYTES

xfstests: rename RETAIN_AG_BYTES

Rename the $RETAIN_AG_BYTES variable to be more generic so that it
reflects the fact that it is designed to retain a certain amount of
extra free space above the default amount in the filesystem when
doing large scratch device testing.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>