lsfd: optimize maps use

lsfd: optimize maps use

The /proc/#/maps file usually contains the same file more than once.
This patch tries to reuse previous file to reduce number of stat()
calls, see "strace --summary-only" output below, the syscall is reduce
in 50%.

old version:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 55.57    0.118708           2     50756      5414 newfstatat

new version:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 39.17    0.058501           2     24436      5382 newfstatat

Signed-off-by: Karel Zak <kzak@redhat.com>