ubi: fastmap: Fix add_vol() return value test in ubi_attach_fastmap()

ubi: fastmap: Fix add_vol() return value test in ubi_attach_fastmap()

Commit 9b51c3a1b0a1 ("UBI: Fastmap: Do not add vol if it already
exists") introduced a bug by changing the possible error codes returned
by add_vol():
- this function no longer returns NULL in case of allocation failure
  but return ERR_PTR(-ENOMEM)
- when a duplicate entry in the volume RB tree is found it returns
  ERR_PTR(-EEXIST) instead of ERR_PTR(-EINVAL)

Fix the tests done on add_vol() return val to match this new behavior.

Fixes: 9b51c3a1b0a1 ("UBI: Fastmap: Do not add vol if it already exists")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>