ocfs2: Wrap extent block reads in a dedicated function.

We weren't consistently checking extent blocks after we read them.
Most places checked the signature, but none checked h_blkno or
h_fs_signature.  Create a toplevel ocfs2_read_extent_block() that does
the read and the validation.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

diff --cc fs/ocfs2/alloc.c
Simple merge

diff --cc fs/ocfs2/alloc.h
Simple merge

diff --cc fs/ocfs2/extent_map.c
Simple merge

diff --cc fs/ocfs2/ocfs2.h
Simple merge