staging/lustre: adapt proc_dir_entry change

In 3.10 merge window, proc_dir_entry is now private to proc. However,
Lustre lprocfs depends heavily on it and its now-gone read_proc_t and
write_proc_t members.

The patch largely changed the fact, and made lprocfs depend on none of
proc_dir_entry private members. All lprocfs callers are converted to
use the new seq_file scheme.

Also lprocfs_srch is removed so that we can drop lprocfs_lock. All callers
are changed to save created pde in proper place.

See https://jira.hpdd.intel.com/browse/LU-3319 for more details.

Signed-off-by: Peng Tao <tao.peng@emc.com>