DAMON sysfs interface is implemented in a single file, sysfs.c, which has
about 2,800 lines of code.  As the interface is hierarchical and some of
the code can be reused by different hierarchies, it would make more sense
to split out the implementation into common parts and different parts in
multiple files.  As the beginning of the work, create files for common
code and move the global mutex for directories modifications protection
into the new file.

Link: https://lkml.kernel.org/r/20221026225943.100429-8-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>