The _SFR_MEM_ADDR() is a macro that returns the address of the argument.
The _SFR_MEM_ADDR() is defined in hardware/tools/avr/avr/include/avr/sfr_defs.h as below.
#define _SFR_MEM_ADDR(sfr) ((uint16_t) &(sfr))
It return the address of the argument.