cifs: new nt status codes from MS-SMB2

MS-SMB2 spec has introduced two new status codes,
STATUS_SERVER_UNAVAILABLE and STATUS_FILE_NOT_AVAILABLE
which are to be treated as retryable errors.

This change adds these to the available mappings and
maps them to Linux errno EAGAIN.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>