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 num_aces

to le16") split struct smb_acl's __le32 num_aces field into __le16

num_aces 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 build_sec_desc(), 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 "ndr_pull_security_descriptor

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/windows_protocols/ms-dtyp/20233ed8-a6c6-4097-aafa-dd545ed24428