fdtable: read, write, close: only execute methods if non-NULL

Only invoke vtable methods read, write, and close if they are
non-NULL.

The close() vtable method is optional, so that should not return
an error if zvfs_close() is called and that method is
unimplemented.

Otherwise, if zvfs_read() or zvfs_write() are called and the
corresponding vtable method is unimplemented, fail setting
errno to EIO.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>