oprofilefs: handle zero-length writes

oprofilefs: handle zero-length writes

Currently in oprofilefs, files that use ulong_fops mis-handle writes of
zero length.  A count of 0 causes oprofilefs_ulong_from_user to return 0
(success), which then leads to oprofile_set_ulong being called to stuff
"value" into file->private_data without it being initialized.

Fix this by moving the check for a zero-length write up into
ulong_write_file.

Signed-off-by: Mike Waychison <mikew@google.com>
Cc: Robert Richter <robert.richter@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>