Clean up using global_dirty_limit wrapper for writeback probe

Clean up using global_dirty_limit wrapper for writeback probe

Move the wrapper around reading of global_dirty_limit to /wrapper/
directory.  Introduce a new kallsyms_lookup_dataptr function for
obtaining the address unchanged and use it in global_dirty_limit
wrapper.  Since the data address is available only if
CONFIG_KALLSYMS_ALL is set, omit the whole probe from building if this
config is missing.

[ Edit by Mathieu Desnoyers: small coding style fixes ]

Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>