selftests/fuse: add the missing symlink caching test

Commit 3e14f079d7a0 ("selftests/fuse: add fuse symlink caching test")
registered fuse_symlink_cache_test in the fuse selftest Makefile and
.gitignore, but the test source was not part of the applied commit, so
building the fuse selftests fails:

  make[1]: *** No rule to make target 'fuse_symlink_cache_test.c', needed by '.../fuse_symlink_cache_test'.

Add fuse_symlink_cache_test.c as posted in Luis Henriques's v5
submission. The test serves a file and a symlink to it from a libfuse
low-level filesystem, resolves the symlink 100 times, and counts the
readlink requests that reach the server: one when FUSE_CAP_CACHE_SYMLINKS
is set during FUSE_INIT, and all 100 when it is not.

Fixes: 3e14f079d7a0 ("selftests/fuse: add fuse symlink caching test")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202610170822.oEJ1Autq-lkp@intel.com/
Link: https://lore.kernel.org/r/20261001164916.15311-8-luis@igalia.com
Originally-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Reviewed-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>