btrfs: use explicit initializer for seq_elem

Using {} as initializer for struct seq_elem does not properly initialize
the list_head member, but it currently works because it gets set through
btrfs_get_tree_mod_seq if 'seq' is 0.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/backref.c
Simple merge

diff --cc fs/btrfs/ctree.h
Simple merge

diff --cc fs/btrfs/ioctl.c
Simple merge

diff --cc fs/btrfs/qgroup.c
Simple merge