libext2fs: don't cache inodes that fail checksum verification

libext2fs: don't cache inodes that fail checksum verification

If an inode fails checksum verification, don't stuff a copy of it in
the inode cache, because this can cause the library to fail to return
the "corrupt inode" error code.

In general, this happens if ext2fs_read_inode_full() is called twice
on an inode with an incorrect checksum.  If fs->flags has
EXT2_FLAG_IGNORE_CSUM_ERRORS set during the first call and *unset*
during the second call, the cache hit during the second call fails to
return EXT2_ET_INODE_CSUM_INVALID as you'd expect.  This happens
during fsck because the first read_inode call happens as part of
check_blocks and the second call happens during inode checksum
revalidation.  A file system with a slightly corrupt non-extent inode
will trigger this.

While we're at it, make the inode read function consistent with the
rest of libext2fs -- copy the metadata object into the caller's buffer
even if it fails checksum verification.  This will help e2fsck avoid a
double re-read later on down the line.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>