scsi: sr: Add memory allocation failure handling for get_capabilities()

The function get_capabilities() has the possibility of failing to allocate
the transfer buffer but it does not currently handle this. This may lead to
exceptions when accessing the buffer.

Add error handling when memory allocation fails.

Link: https://lore.kernel.org/r/20220427025647.298358-1-lienze@kylinos.cn


Signed-off-by: Enze Li <lienze@kylinos.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>