ANDROID: sdcardfs: fix potential crash when reserved_mb is not zero

sdcardfs_mkdir() calls check_min_free_space(). When reserved_mb is not zero,
a negative dentry will be passed to ext4_statfs() at last and ext4_statfs()
will crash. The parent dentry is positive. So we use the parent dentry to
check free space.

Change-Id: I80ab9623fe59ba911f4cc9f0e029a1c6f7ee421b
Signed-off-by: Lianjun Huang <huanglianjun@vivo.com>