mm/damon/sysfs: prohibit multiple physical address space monitoring targets

Having multiple targets for physical address space monitoring makes no
sense.  This commit prohibits such a ridiculous DAMON context setup my
making the DAMON context build function to check and return an error for
the case.

Link: https://lkml.kernel.org/r/20220429160606.127307-7-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>