btrfs-progs: fix bug on mkfs with relative path specified

btrfs-progs: fix bug on mkfs with relative path specified

The bug accurs when exec:
	# mkfs.btrfs -r <a relative path> <device>
	(note: the path should be 'valid' correspond to your `pwd`)
error msg:
	$ scandir for <a relative path> failed: No such file...

o Replace strdup() with realpath() to get the correct scan path.
o fix memory leaks and adopt the "single return + goto out" pattern

Reported-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>