btrfs: properly flag filesystem with BTRFS_FEATURE_INCOMPAT_BIG_METADATA

Commit 6f93e834 seemingly inadvertently moved the code responsible
for flagging the filesystem as having BIG_METADATA to a place where
setting the flag was essentially lost. This means that
filesystems created with kernels containing this bug (starting with 5.15)
can potentially be mounted by older (pre-3.4) kernels. In reality
chances for this happening are low because there are other incompat
flags introduced in the mean time. Still the correct behavior is to set
INCOMPAT_BIG_METADATA flag and persist this in the superblock.

Fixes: 6f93e834 ("btrfs: fix upper limit for max_inline for page size 64K")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>