f2fs: avoid format-overflow warning

In the Linux kernel, the following vulnerability has been resolved:

f2fs: avoid format-overflow warning

With gcc and W=1 option, there's a warning like this:

fs/f2fs/compress.c: In function ‘f2fs_init_page_array_cache’:
fs/f2fs/compress.c:1984:47: error: ‘%u’ directive writing between
1 and 7 bytes into a region of size between 5 and 8
[-Werror=format-overflow=]
1984 | sprintf(slab_name, "f2fs_page_array_entry-%u:%u", MAJOR(dev),
MINOR(dev));
| ^~

String "f2fs_page_array_entry-%u:%u" can up to 35. The first "%u" can up
to 4 and the second "%u" can up to 7, so total size is "24 + 4 + 7 = 35".
slab_name's size should be 35 rather than 32.