fs/sysv: Change the signature of dir_get_page()

Change the signature of dir_get_page() in order to prepare this function
to the conversion to the use of kmap_local_page(). Change also those call
sites which are required to adjust to the new signature.

Cc: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>