xfs: More robust inode extent count validation

When the inode is in extent format, it can't have more extents that
fit in the inode fork. We don't currenty check this, and so this
corruption goes unnoticed by the inode verifiers. This can lead to
crashes operating on invalid in-memory structures.

Attempts to access such a inode will now error out in the verifier
rather than allowing modification operations to proceed.

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix a typedef, add some braces and breaks to shut up compiler warnings]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>