Copying to a 16 byte structure into an 8-byte struct member
causes a compile-time warning:

 | In file included from drivers/firmware/arm_ffa/driver.c:25:
 | In function 'fortify_memcpy_chk',
 |    inlined from 'export_uuid' at include/linux/uuid.h:88:2,
 |    inlined from 'ffa_msg_send_direct_req2' at drivers/firmware/arm_ffa/driver.c:488:2:
 | include/linux/fortify-string.h:571:25: error: call to '__write_overflow_field'
 |   declared with attribute warning: detected write beyond size of field
 |   (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
 |                         __write_overflow_field(p_size_field, size);

Use a union for the conversion instead and make sure the byte order
is fixed in the process.

Fixes: aaef3bc9 ("firmware: arm_ffa: Add support for FFA_MSG_SEND_DIRECT_{REQ,RESP}2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <20240909110938.247976-1-arnd@kernel.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>