Kernel: Move symlink recursion limit to .h, increase it to 8

As pointed out by BertalanD on Discord, POSIX specifies that
_SC_SYMLOOP_MAX (implemented in the following commit) always needs to be
equal or more than _POSIX_SYMLOOP_MAX (8, defined in
LibC/bits/posix1_lim.h), hence I've increased it to that value to
comply with the standard.

The move to header is required for the following commit - to make this
constant accessible outside of the VFS class, namely in sysconf.

Kernel/FileSystem/VirtualFileSystem.h

@@ -35,6 +35,10 @@ struct UidAndGid {
 class VirtualFileSystem {
     AK_MAKE_ETERNAL
 public:
+    // Required to be at least 8 by POSIX
+    // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html
+    static constexpr int symlink_recursion_limit = 8;
+
     static void initialize();
     static VirtualFileSystem& the();