gfs2: Don't return NULL from gfs2_inode_lookup

Callers expect gfs2_inode_lookup to return an inode pointer or ERR_PTR(error).
Commit b66648ad caused it to return NULL instead of ERR_PTR(-ESTALE) in
some cases.  Fix that.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: b66648ad ("gfs2: Move inode generation number check into gfs2_inode_lookup")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>