Add semantic checks to btrfsck for files and directories

Add semantic checks to btrfsck for files and directories

This patch makes btrfsck check more things, including
directory items, file extents, checksumming, inode link
counts etc.

The code for these checks is similar to the code verifies
extent back references. The main difference is that
shared tree blocks are treated specially. The partial
checking results(unresolved references and/or errors)
of shared sub-trees are cached. This avoids scanning
the shared blocks several times. Thank you,

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>