docs-rst: filesystems: use c domain references where needed

docs-rst: filesystems: use c domain references where needed

Instead of just mention the function names, use cross-references
to the kernel-doc tags where pertinent.

While not all function documentation is included here, I
double-checked that all functions mentioned there still
exists.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>