lslocks: special-case PID for filtering (skip −1)

lslocks: special-case PID for filtering (skip −1)

The PID column may be printed as −1 for locks that are not tied to a
process or are inaccessible to lslocks (e.g., permission constraints).
The libsmartcols filter engine does not support negative integers, so
PID = −1 cannot be compared reliably.

Attach the PID as cell userdata only when PID >= 0, and use that value
for filter evaluation. Rows with PID = −1 are thus excluded from PID
comparisons.

No change to printed output.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>