CVE-2026-46139: smb: client: use kzalloc to zero-initialize security descriptor buffer

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

smb: client: use kzalloc to zero-initialize security descriptor buffer

Commit 62e7dd0a39c2d ("smb: common: change the data type of numaces to le16") split struct smbacl's le32 numaces field into le16 numaces and le16 reserved. The reserved field corresponds to Sbz2 in the MS-DTYP ACL wire format, which must be zero [1].

When building an ACL descriptor in buildsecdesc(), we are using a kmalloc()'ed descriptor buffer and writing the fields explicitly using le16() writes now. This never writes to the 2 byte reserved field, leaving it as uninitialized heap data.

When the reserved field happens to contain non-zero slab garbage, Samba rejects the security descriptor with "ndrpullsecuritydescriptor failed: Range Error", causing chmod to fail with EINVAL.

Change kmalloc() to kzalloc() to ensure the entire buffer is zero-initialized.

[1] https://learn.microsoft.com/en-us/openspecs/windowsprotocols/ms-dtyp/20233ed8-a6c6-4097-aafa-dd545ed24428