lsfd: refer to /proc/$pid/map_files if a mapped file is masked

lsfd: refer to /proc/$pid/map_files if a mapped file is masked

To extract file information, lsfd called stat(2) on the the file name
found in /proc/$pid/maps. However, lsfd may retrieve information for
the wrong file if the file is bind-mount'ed after the process maps the
file.

To obtain the information about the originally mapped file, this
change makes lsfd call fstat(2) on the corresponding entry under
/proc/$pid/map_files avoiding the bind-mount ambiguity.

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