lib/rmobj: avoid using lstat

rmobj() is failing for _64 version of testcases, which use large
files, where it hits EOVERFLOW in lstat():

open12      0  TWARN  :  tst_tmpdir.c:206: tst_rmdir: rmobj(/tmp/opexIqjV3) failed:
       lstat(/tmp/opexIqjV3/large_file) failed; errno=75:
       Value too large for defined data type

Avoid using any *stat functions, instead try to open "obj"
with open(O_DIRECTORY | O_NOFOLLOW,..). If it succeeds,
we know we are dealing with directory, otherwise assume
it's a file or symlink.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>