posix: fs: ZFD_IOCTL_CLOSE: Be sure to call posix_fs_free_obj()

To make sure that entry in fs.c:desc_array[] is freed. Note that
freeing an entry in fdtable is handled by generic implementation
of close().

Fixes: #17231

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>