libmount: Use filter arg of scandir* to pickup /etc/fstab.d/*.fstab files

libmount: Use filter arg of scandir* to pickup /etc/fstab.d/*.fstab files

This patch is based on my last patch:

     [PATCH 1/2] scandirat based mnt_table_parse_dir implementation

scandir* used in `mnt_table_parse_dir in libmount/src/tab_parse.c
can take filter function as an argument.

`mnt_table_parse_dir' picks up fstab files from namelist returned from
scandir* for itself. However, some parts of picking-up job can be done
in the filter function.

This patch introduces a new function `mnt_table_parse_dir_filter' to
share the code for picking-up job between two implementations of
`mnt_table_parse_dir_filter', scandir based and scandirat based.

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