CVE-2026-52956: libceph: Fix potential out-of-bounds access in __ceph_x_decrypt()

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

libceph: Fix potential out-of-bounds access in cephxdecrypt()

In cephxdecrypt(), a part of the buffer p is interpreted as a cephxencryptheader, and the magic field of this struct is accessed. This happens without any guarantee that the buffer is large enough to hold this struct. The function parameter ciphertextlen represents the length of the ciphertext to decrypt and is guaranteed to be at most the remaining size of the allocated buffer p. However, this value is not necessarily greater than sizeof(cephxencryptheader). E.g., a message frame of type FRAMETAGAUTHREPLYMORE, that is just as long to hold the ciphertext at its end with a ciphertextlen of 8 or less, can trigger an out-of-bounds memory access when accessing hdr->magic.

This patch fixes the issue by adding a check to ensure that the decrypted plaintext in the buffer is large enough to represent at least the cephxencryptheader.