repart: silence bogus gcc warning

repart: silence bogus gcc warning

[2/3] Compiling C object systemd-repart.p/src_partition_repart.c.o
../src/partition/repart.c: In function ‘context_open_copy_block_paths’:
../src/partition/repart.c:5194:41: warning: ‘devno’ may be used uninitialized [-Wmaybe-uninitialized]
 5194 |                         source_fd = r = device_open_from_devnum(S_IFBLK, devno, O_RDONLY|O_CLOEXEC|O_NONBLOCK, &opened);
      |                                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/partition/repart.c:5188:31: note: ‘devno’ was declared here
 5188 |                         dev_t devno;
      |                               ^~~~~

This is with gcc-13.0.1-0.2.fc38.x86_64, -O2. I'm pretty sure the code
is correct. I also tried adding some asserts where errno is used for the return
value, but that didn't help. I think resolve_copy_blocks_auto() is just too long
for gcc to understand.