fix the name_to_dev_t mess v2

[Christoph Hellwig]

Hi all,

this series tries to sort out accumulated mess around the name_to_dev_t
function.  This function is intended to allow looking up the dev_t of a
block device based on a name string before the root file systems is
mounted and thus the normal path based lookup is available.

Unfortunately a few years ago it managed to get exported and used in
non-init contexts, leading to the something looking like a path name
also beeing lookuped up by a different and potential dangerous
algorithm.

This series does a fair amount of refactoring and finally ends up with
the renamed and improved name_to_dev_t only beeing available for the
early init code again.

The series is against Jens' for-6.5/block tree but probably applies
against current mainline just fine as well.

A git tree is also available here:

    git://git.infradead.org/users/hch/block.git blk-init-cleanup

Gitweb:

    http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/blk-init-cleanup

Changes since v1:
 - really propagate the actual error in dm_get_device
 - improve the documentation in kernel-parameters.txt
 - spelling fixes

Diffstat: