Update tee_ta_manager.c

Fix user heap corruption by usr_params

Avoids corruption of heap by usr_params, and vice versa, when 
stack_size+heap_size is a multiple of 1 MiB. usr_params was
previously unintentionally stored just outside the allocated
stack. If there is not enough padding between stack and heap,
usr_params will overwrite a part of the heap.
Not exactly, Take one example：
if a share parameter with p->memref.buffer = 0x500000 and p->memref.size = 0x1000
with the original code the nbr_sections=6

the follow loop will exec 6 times mapped 6 sections
while (section < nbr_sections) {
**buffer =
((section + py_offset) << SECTION_SHIFT) |
sect_prot;
(*buffer)++;
section++;
}

indeed we only need to map one section.

By the way , the tee_mmu_is_mapped also has such issue.