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

In the Linux kernel, the following vulnerability has been resolved:

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

The commit message of commit 6ec1f0239485 ("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.