btrfs: zoned: calculate max_extent_size properly on non-zoned setup

Since commit 559218d4 ("block: pre-calculate max_zone_append_sectors"),
queue_limits's max_zone_append_sectors is default to be 0 and it is only
updated when there is a zoned device. So, we have
lim->max_zone_append_sectors = 0 when there is no zoned device in the
filesystem.

That leads to fs_info->max_zone_append_size and thus
fs_info->max_extent_size to be 0, which is wrong and can for example
lead to a divide by zero in count_max_extents().

Fix this by only capping fs_info->max_extent_size to
fs_info->max_zone_append_size when it is non-zero.

Based on a patch from Naohiro Aota <naohiro.aota@wdc.com>, from which
much of this commit message is stolen as well.

Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Fixes: 559218d4 ("block: pre-calculate max_zone_append_sectors")
Tested-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>