munmap() should not fail when mm_struct.map_count temporarily reaches max_map_count

Issue

• The munmap() syscall fails even  if the number of VMAs of a process (mm_struct.map_count) does not yet  reach the upper limit (sysctl_max_map_count).  The munmap() fails with ENOMEM if the number of VMAs equals the "limit - 1"  and it does not unmap a whole VMA but a part of a VMA.

• In that case, the number of VMAs temporarily reaches the limit during munmap() and it backs to below the limit at the end of munmap(). So it
    should not fail.