fs/smb/client: implement chmod() for SMB3 POSIX Extensions

The NT ACL format for an SMB3 POSIX Extensions chmod() is a single ACE with the
magic S-1-5-88-3-mode SID:

  NT Security Descriptor
      Revision: 1
      Type: 0x8004, Self Relative, DACL Present
      Offset to owner SID: 56
      Offset to group SID: 124
      Offset to SACL: 0
      Offset to DACL: 20
      Owner: S-1-5-21-3177838999-3893657415-1037673384-1000
      Group: S-1-22-2-1000
      NT User (DACL) ACL
          Revision: NT4 (2)
          Size: 36
          Num ACEs: 1
          NT ACE: S-1-5-88-3-438, flags 0x00, Access Allowed, mask 0x00000000
              Type: Access Allowed
              NT ACE Flags: 0x00
              Size: 28
              Access required: 0x00000000
              SID: S-1-5-88-3-438

Owner and Group should be NULL, but the server is not required to fail the
request if they are present.

Signed-off-by: Ralph Boehme <slow@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>