LU-20 osd-ldiskfs: Make readonly patches optional

LU-20 osd-ldiskfs: Make readonly patches optional

If CONFIG_KALLSYMS is enabled, see if there are dev_set/check_rdonly
functions present and use them if so.
Otherwise assume the functionality is unavailable.

This should allow using osd_ldiskfs module on both patched and
unpatched kernels with no modifications thanks to weak updates.

Change-Id: I274db77cf76abc63ad6a5643a98c521778f294b3
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: https://review.whamcloud.com/27549
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>