x86 early_ioremap: Increase FIX_BTMAPS_SLOTS to 8

3.16 kernel boot fail with earlyprintk=efi, it keeps scrolling at the
bottom line of screen.

Bisected, the first bad commit is below:
commit 86dfc6f3
Author: Lv Zheng <lv.zheng@intel.com>
Date:   Fri Apr 4 12:38:57 2014 +0800

    ACPICA: Tables: Fix table checksums verification before installation.

I did some debugging by enabling both serial and efi earlyprintk, below is
some debug dmesg, seems early_ioremap fails in scroll up function due to
no free slot, see below dmesg output:

  WARNING: CPU: 0 PID: 0 at mm/early_ioremap.c:116 __early_ioremap+0x90/0x1c4()
  __early_ioremap(ed00c800, 00000c80) not found slot
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 3.17.0-rc1+ #204
  Hardware name: Hewlett-Packard HP Z420 Workstation/1589, BIOS J61 v03.15 05/09/2013
  Call Trace:
    dump_stack+0x4e/0x7a
    warn_slowpath_common+0x75/0x8e
    ? __early_ioremap+0x90/0x1c4
    warn_slowpath_fmt+0x47/0x49
    __early_ioremap+0x90/0x1c4
    ? sprintf+0x46/0x48
    early_ioremap+0x13/0x15
    early_efi_map+0x24/0x26
    early_efi_scroll_up+0x6d/0xc0
    early_efi_write+0x1b0/0x214
    call_console_drivers.constprop.21+0x73/0x7e
    console_unlock+0x151/0x3b2
    ? vprintk_emit+0x49f/0x532
    vprintk_emit+0x521/0x532
    ? console_unlock+0x383/0x3b2
    printk+0x4f/0x51
    acpi_os_vprintf+0x2b/0x2d
    acpi_os_printf+0x43/0x45
    acpi_info+0x5c/0x63
    ? __acpi_map_table+0x13/0x18
    ? acpi_os_map_iomem+0x21/0x147
    acpi_tb_print_table_header+0x177/0x186
    acpi_tb_install_table_with_override+0x4b/0x62
    acpi_tb_install_standard_table+0xd9/0x215
    ? early_ioremap+0x13/0x15
    ? __acpi_map_table+0x13/0x18
    acpi_tb_parse_root_table+0x16e/0x1b4
    acpi_initialize_tables+0x57/0x59
    acpi_table_init+0x50/0xce
    acpi_boot_table_init+0x1e/0x85
    setup_arch+0x9b7/0xcc4
    start_kernel+0x94/0x42d
    ? early_idt_handlers+0x120/0x120
    x86_64_start_reservations+0x2a/0x2c
    x86_64_start_kernel+0xf3/0x100

Quote reply from Lv.zheng about the early ioremap slot usage in this case:

"""
In early_efi_scroll_up(), 2 mapping entries will be used for the src/dst screen buffer.
In drivers/acpi/acpica/tbutils.c, we've improved the early table loading code in acpi_tb_parse_root_table().
We now need 2 mapping entries:
1. One mapping entry is used for RSDT table mapping. Each RSDT entry contains an address for another ACPI table.
2. For each entry in RSDP, we need another mapping entry to map the table to perform necessary check/override before installing it.

When acpi_tb_parse_root_table() prints something through EFI earlyprintk console, we'll have 4 mapping entries used.
The current 4 slots setting of early_ioremap() seems to be too small for such a use case.
"""

Thus increase the slot to 8 in this patch to fix this issue.
boot-time mappings become 512 page with this patch.

Signed-off-by: Dave Young <dyoung@redhat.com>
Cc: <stable@vger.kernel.org> # v3.16
Signed-off-by: Matt Fleming <matt.fleming@intel.com>