xfs: start gc on zonegc_low_space attribute updates

xfs: start gc on zonegc_low_space attribute updates

Start gc if the agressiveness of zone garbage collection is changed
by the user (if the file system is not read only).

Without this change, the new setting will not be taken into account
until the gc thread is woken up by e.g. a write.

Cc: stable@vger.kernel.org # v6.15
Fixes: 845abeb1f06a8a ("xfs: add tunable threshold parameter for triggering zone GC")
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>