powerpc/fadump: Set an upper limit for boot memory size

By default, 5% of system RAM is reserved for preserving boot memory.
Alternatively, a user can specify the amount of memory to reserve.
See Documentation/powerpc/firmware-assisted-dump.txt for details. In
addition to the memory reserved for preserving boot memory, some more
memory is reserved, to save HPTE region, CPU state data and ELF core
headers.

Memory Reservation during first kernel looks like below:

  Low memory                                        Top of memory
  0      boot memory size                                       |
  |           |                       |<--Reserved dump area -->|
  V           V                       |   Permanent Reservation V
  +-----------+----------/ /----------+---+----+-----------+----+
  |           |                       |CPU|HPTE|  DUMP     |ELF |
  +-----------+----------/ /----------+---+----+-----------+----+
        |                                           ^
        |                                           |
        \                                           /
         -------------------------------------------
          Boot memory content gets transferred to
          reserved area by firmware at the time of
          crash

This implicitly means that the sum of the sizes of boot memory, CPU
state data, HPTE region, DUMP preserving area and ELF core headers
can't be greater than the total memory size. But currently, a user is
allowed to specify any value as boot memory size. So, the above rule
is violated when a boot memory size around 50% of the total available
memory is specified. As the kernel is not handling this currently, it
may lead to undefined behavior. Fix it by setting an upper limit for
boot memory size to 25% of the total available memory. Also, instead
of using memblock_end_of_DRAM(), which doesn't take the holes, if any,
in the memory layout into account, use memblock_phys_mem_size() to
calculate the percentage of total available memory.

Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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