libext2fs: correctly subtract xattr blocks on bigalloc filesystems

libext2fs: correctly subtract xattr blocks on bigalloc filesystems

ext2fs_inode_data_blocks2() calculates an inode's data block count by
subtracting the external xattr block, if any, from the total blocks.
But on bigalloc filesystems, the xattr "block" is actually a whole
cluster, so ext2fs_inode_data_blocks2() would return a too-large value.

It seems this could have caused several different problems, but the one
I encountered was that xfstest generic/399 failed in the "bigalloc"
config because e2fsck incorrectly considered a symlink on the filesystem
to be corrupted at the end of the test.  This happened because e2fsck
incorrectly calculated a nonzero data block count for a "fast" symlink
with an external xattr block and therefore treated it as a "slow"
symlink, which failed validation.

Fix this by updating ext2fs_inode_data_blocks2() to subtract the cluster
size rather than the block size.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>