proc: task_state: deuglify the max_fds calculation

proc: task_state: deuglify the max_fds calculation

1. The usage of fdt looks very ugly, it can't be NULL if ->files is
   not NULL. We can use "unsigned int max_fds" instead.

2. This also allows to move seq_printf(max_fds) outside of task_lock()
   and join it with the previous seq_printf(). See also the next patch.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Aaron Tomlin <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>,
Cc: Sterling Alexander <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Roland McGrath <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>