fstests: fix compilation error in splice2pipe on old (<C99) compilers

fstests: fix compilation error in splice2pipe on old (<C99) compilers

Compilation fails on system with compiler which uses older C dialect
(e.g. RHEL7; gcc with gnu90) with:

splice2pipe.c: In function 'prepare_pipe':
splice2pipe.c:54:2: error: 'for' loop initial declarations are only allowed in C99 mode
  for (unsigned r = pipe_size; r > 0;) {

Fix it by declaring 'r' outside of the loop.

Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>