Revert "fs/9p: Refresh metadata in d_revalidate for uncached mode too"

This reverts commit 290434474c332a2ba9c8499fe699c7f2e1153280.

That commit broke cache=mmap, a mode that doesn't cache metadata,
but still has writeback cache.

In commit 290434474c33 ("fs/9p: Refresh metadata in d_revalidate
for uncached mode too") we considered metadata cache to be enough to
not look at the server, but in writeback cache too looking at the server
size would make the vfs consider the file has been truncated before the
data has been flushed out, making the following repro fail (nothing is
ever read back, the resulting file ends up with no data written)
```
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

char buf[4096];

int main(int argc, char *argv[])
{
int ret, i;
int fdw, fdr;

if (argc < 2)
return 1;

fdw = openat(AT_FDCWD, argv[1], O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 0600);
if (fdw < 0) {
fprintf(stderr, "cannot open fdw\n");
return 1;
}
write(fdw, buf, sizeof(buf));

fdr = openat(AT_FDCWD, argv[1], O_RDONLY|O_CLOEXEC);

if (fdr < 0) {
fprintf(stderr, "cannot open fdr\n");
close(fdw);
return 1;
}

for (i = 0; i < 10; i++) {
ret = read(fdr, buf, sizeof(buf));
fprintf(stderr, "i: %d, read returns %d\n", i, ret);
}

close(fdr);
close(fdw);
return 0;
}
```

There is a fix for this particular reproducer but it looks like there
are other problems around metadata refresh (e.g. around file rename), so
revert this to avoid d_revalidate in uncached mode for now.

Reported-by: Song Liu <song@kernel.org>
Link: https://lkml.kernel.org/r/CAHzjS_u_SYdt5=2gYO_dxzMKXzGMt-TfdE_ueowg-Hq5tRCAiw@mail.gmail.com
Reported-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Link: https://lore.kernel.org/bpf/CAEf4BzZbCE4tLoDZyUf_aASpgAGFj75QMfSXX4a4dLYixnOiLg@mail.gmail.com/
Fixes: 290434474c33 ("fs/9p: Refresh metadata in d_revalidate for uncached mode too")
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>