Kernel/SysFS: Provide a way to "truncate" and "set" mtime on inodes

Normally, trying to truncate a SysFSInode should result in EPERM error.
However, as suggested by Ali (@alimpfard), we can allow the PowerState
node to be "truncated" so one can open that file with O_TRUNC option.
Likewise, we also need to provide a way to set modified time on SysFS
inodes. For most inodes, we should return ENOTIMPL error, but for the
power state switch, we ignore the modified time setting and just return
KSuccess.

These fixes allow to do "echo -n 1 > /sys/firmware/power_state" in Shell
after gaining root permissions, to switch the power state.

Kernel/FileSystem/SysFS.h

@@ -146,6 +146,7 @@ protected:
     virtual KResult chmod(mode_t) override;
     virtual KResult chown(UserID, GroupID) override;
     virtual KResult truncate(u64) override;
+    virtual KResult set_mtime(time_t);
 
     virtual KResult attach(OpenFileDescription& description) override final;
     virtual void did_seek(OpenFileDescription&, off_t) override final;