zoned GC fix v2

Hi Carlos,

this series fixes a potential deadlock in the zoned GC code, and
also adds a comment in the same area while looking at it.

Changes since v1:
 - improve a commit log
 - improve a code commit

Diffstat:
 xfs_zone_gc.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)