drivers: staging: lustre: Use mult if units not specified

Units can be passed to lprocfs_write_frac_u64_helper() via a suffix
(e.g., "...K", "...M", etc.) tacked onto the value.  A comment states
that "specified units override the multiplier," though the multiplier is
overridden regardless.  Update the conditional logic so that it only
applies when units are specified.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
drivers: staging: lustre: Track sign separately

The `mult' parameter is negated if the user data begins with a '-' so
that the final value has the appropriate sign.  But `mult' is only used
if the user data does not include a "units" suffix.  In this case,
`mult' is overridden with the numeric scale conveyed by the units suffix,
but retains the sign of the original value.

Having `mult' serving double-duty works but is confusing.  Use a new
local variable to store the sign of the user data instead.  This also
fixes a pitfall of passing 0 to `mult', expecting it to be ignored when
a units suffix is specified, but having the effect of taking the
absolute value of the user-provided data.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>