Btrfs-progs: check return value of realpath(3)

Btrfs-progs: check return value of realpath(3)

I hit a segfault when deleting a subvolume with very long name(>4096),
it's because cmd_subvol_delete() calls strdup() and passes NULL as
argument, which is returned by realpath(3).

I used the following script to reproduce

	#!/bin/bash
	mnt=$1
	i=1
	path=$mnt/subvol_$i

	# Create very deep subvolumes
	while btrfs sub create $path;do
	      ((i++))
	      path="$path/subvol_$i"
	done
	last_vol=$(dirname $path)
	dir=$(dirname $last_vol)
	vol=$(basename $last_vol)

	# Try to delete tha last one, this would get segfault
	pushd $dir
	btrfs sub delete $vol
	popd

Fix it by checking return value of realpath(3), also fix the one in
find_mount_root().

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>