ksmbd: retrieve number of blocks using vfs_getattr in set_file_allocation_info

Use vfs_getattr() to retrieve stat information, rather than make
assumptions about how a filesystem fills inode structs.

Cc: stable@vger.kernel.org
Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>