md/md-bitmap: fix GPF in bitmap_get_stats()

The commit message of commit 6ec1f023 ("md/md-bitmap: fix stats
collection for external bitmaps") states:

    Remove the external bitmap check as the statistics should be
    available regardless of bitmap storage location.

    Return -EINVAL only for invalid bitmap with no storage (neither in
    superblock nor in external file).

But, the code does not adhere to the above, as it does only check for
a valid super-block for "internal" bitmaps. Hence, we observe:

Oops: GPF, probably for non-canonical address 0x1cd66f1f40000028
RIP: 0010:bitmap_get_stats+0x45/0xd0
Call Trace:

 seq_read_iter+0x2b9/0x46a
 seq_read+0x12f/0x180
 proc_reg_read+0x57/0xb0
 vfs_read+0xf6/0x380
 ksys_read+0x6d/0xf0
 do_syscall_64+0x8c/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

We fix this by checking the existence of a super-block for both the
internal and external case.

Fixes: 6ec1f023 ("md/md-bitmap: fix stats collection for external bitmaps")
Cc: stable@vger.kernel.org
Reported-by: Gerald Gibson <gerald.gibson@oracle.com>
Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Link: https://lore.kernel.org/linux-raid/20250702091035.2061312-1-haakon.bugge@oracle.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>