fs: omfs: Use flexible-array member in struct omfs_extent

fs: omfs: Use flexible-array member in struct omfs_extent

Memory for 'struct omfs_extent' and a 'e_extent_count' number of extent
entries is indirectly allocated through 'bh->b_data', which is a pointer
to data within the page. This implies that the member 'e_entry'
(which is the start of extent entries) functions more like an array than
a single object of type 'struct omfs_extent_entry'.

So we better turn this object into a proper array, in this case a
flexible-array member, and with that, fix the following
-Wstringop-overflow warning seen after building s390 architecture with
allyesconfig (GCC 13):

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 */
      |                                  ^~~~~~~

There are some binary differences before and after changes, but this are
expected due to the change in the size of 'struct omfs_extent' and the
necessary adjusments.

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/330
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>