xfs: avoid unnecessary calculations in xfs_zoned_need_gc()

xfs: avoid unnecessary calculations in xfs_zoned_need_gc()

If zonegc_low_space is set to zero (which is the default), the second
condition in xfs_zoned_need_gc() that triggers GC never evaluates to
true because the calculated threshold will always be 0. So there is no
need to calculate the threshold and to evaluate that condition. Return
early when zonegc_low_space is zero.

While at it, add comments to document the intent of each of the 3 tests
used to determine the return value to control the execution of garbage
collection.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>