some fix about snapfs

some fix about snapfs
1)add clonefs check before snap delete
2)some fix about orphan list
3)initialize inode in mknod, mkdir symlink ...
4)some fix about symlinks
5)add some snapshot filter operations (listxattr, getxattr)
6)some fix about sparse files
7)fix page buffer problems in snap delete and snap_do_cow
8)adding some functions  in snapconf.
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