Fix -Wstringop-overflow warnings in fs/omfs/file.c

[GustavoARSilva]

Seen after building s390 with allyesconfig:

fs/omfs/file.c: In function 'omfs_grow_extent':
include/linux/fortify-string.h:57:33: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
   57 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
include/linux/fortify-string.h:648:9: note: in expansion of macro '__underlying_memcpy'
  648 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
include/linux/fortify-string.h:693:26: note: in expansion of macro '__fortify_memcpy_chk'
  693 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
fs/omfs/file.c:170:9: note: in expansion of macro 'memcpy'
  170 |         memcpy(terminator, entry, sizeof(struct omfs_extent_entry));
      |         ^~~~~~
In file included from fs/omfs/omfs.h:8,
                 from fs/omfs/file.c:11:
fs/omfs/omfs_fs.h:80:34: note: at offset 16 into destination object 'e_entry' of size 16
   80 |         struct omfs_extent_entry e_entry;       /* start of extent entries */
      |                                  ^~~~~~~

[GustavoARSilva]

Patch: https://lore.kernel.org/linux-hardening/ZLGodUeD307GlINN@work/

[GustavoARSilva]

fixed in commit 4d8cbf6dbcdaebe949461b0a933ae4c71cb53edc