dynamic_debug: add trim_prefix() to provide source-root relative paths

trim_prefix(path) skips past the absolute source path root, and
returns the pointer to the relative path from there.  It is used to
shorten the displayed path in $DBGMT/dynamic_debug/control via
ddebug_proc_show(), and in ddebug_change() to allow relative filenames
to be used in applied queries.  For example:

  ~# echo file kernel/freezer.c +p > $DBGMT/dynamic_debug/control

  kernel/freezer.c:128 [freezer]cancel_freezing p "  clean up: %s\012"

trim_prefix(path) insures common prefix before trimming it, so
out-of-tree module paths are shown as full absolute paths.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>