libext2fs: fix ext2fs_link() when the directory has an extent tree depth > 1

libext2fs: fix ext2fs_link() when the directory has an extent tree depth > 1

Ext2fs_link() was passing the wrong inode number to ext2fs_bmap(); as
a result, when a directory inode was using extents and the extent tree
depth was greater than 1, the extent tree checksum would be
incorrectly calculated resulting in a error that the extent tree block
checksum was incorrect.

Fixes: 53aa6c54224f ("libext2fs: add the EXT2FS_LINK_APPEND flag ...)
Addresses-Debian-Bug: #1106854
Signed-off-by: Theodore Ts'o <tytso@mit.edu>