xfs: simplify iext overflow checking and upgrade

Currently the calls to xfs_iext_count_may_overflow and
xfs_iext_count_upgrade are always paired.  Merge them into a single
function to simplify the callers and the actual check and upgrade
logic itself.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>