exportfs: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

exportfs: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members instead. So, replace zero-length array
declaration in struct fid with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for a flexible-array member in a union.

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/197
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>