MIPS: Trivial style cleanups in mmap.c

Fix checkpatch warnings.  Rename arch_get_unmapped_area_foo() to
arch_get_unmapped_area_common().  Make indentations and spacing more
consistent.  Add <linux/compiler.h> for likely/unlikely.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: Jian Peng <jipeng2005@gmail.com>
Cc: David Daney <ddaney@caviumnetworks.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2506/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>