ocfs2: fix buffer head management in ocfs2_read_blocks()

In the Linux kernel, the following vulnerability has been resolved: ocfs2: fix buffer head management in ocfs2_read_blocks() In ocfs2_read_blocks(), caller should't assume that buffer head returned by 'sb_getblk()' is exclusively owned and so 'put_bh()' always drops b_count from 1 to 0. If it is not so, buffer head remains on hold and likely to be returned by the next call to 'sb_getblk()' unchanged - that is, with BH_Uptodate bit set even if it has failed validation previously, thus allowing to insert that buffer head into OCFS2 metadata cache and submit it to upper layers. To avoid such a scenario, BH_Uptodate should be cleared immediately after 'validate()' callback has detected some data inconsistency.