fs: Add fchmodat2()

On the userspace side fchmodat(3) is implemented as a wrapper
function which implements the POSIX-specified interface. This
interface differs from the underlying kernel system call, which does not
have a flags argument. Most implementations require procfs [1][2].

There doesn't appear to be a good userspace workaround for this issue
but the implementation in the kernel is pretty straight-forward.

The new fchmodat2() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag,
unlike existing fchmodat.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35
[2] https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28



Co-developed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <f2a846ef495943c5d101011eebcf01179d0c7b61.1689092120.git.legion@kernel.org>
[brauner: pre reviews, do flag conversion in do_fchmodat() directly]
Signed-off-by: Christian Brauner <brauner@kernel.org>