fs/compat.c: fix build on MIPS/s390

The definition of PAGE_CACHE_MASK in <linux/pagemap.h> is needed to use
MAX_RW_COUNT, and on x86-64 that gets done indirectly through the
architecture header includes.  But on MIPS and s390 that doesn't happen,
and we need to make sure that fs/compat.c includes pagemap.h explicitly.

Introduced in commit 435f49a5 ("readv/writev: do the same
MAX_RW_COUNT truncation that read/write does").

Reported-by: Sachin Sant <sachinp@in.ibm.com> (S390)
Reported-by: wu zhangjin <wuzhangjin@gmail.com> (MIPS)
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>