gfs2: Call unlock_new_inode before d_instantiate

gfs2: Call unlock_new_inode before d_instantiate

As Neil Brown describes in detail in the link referenced below, new
inodes must be unlocked before they can be instantiated.

An even better fix is to use d_instantiate_new(), which combines
d_instantiate() and unlock_new_inode().

Fixes: 3d36e57ff768 ("gfs2: gfs2_create_inode rework")
Reported-by: syzbot+0ea5108a1f5fb4fcc2d8@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-fsdevel/177153754005.8396.8777398743501764194@noble.neil.brown.name/
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>