block: clean up blkdev_get() wrappers and their users

After recent blkdev_get() modifications, open_by_devnum() and
open_bdev_exclusive() are simple wrappers around blkdev_get().
Replace them with blkdev_get_by_dev() and blkdev_get_by_path().

blkdev_get_by_dev() is identical to open_by_devnum().
blkdev_get_by_path() is slightly different in that it doesn't
automatically add %FMODE_EXCL to @mode.

All users are converted.  Most conversions are mechanical and don't
introduce any behavior difference.  There are several exceptions.

* btrfs now sets FMODE_EXCL in btrfs_device->mode, so there's no
  reason to OR it explicitly on blkdev_put().

* gfs2, nilfs2 and the generic mount_bdev() now set FMODE_EXCL in
  sb->s_mode.

* With the above changes, sb->s_mode now always should contain
  FMODE_EXCL.  WARN_ON_ONCE() added to kill_block_super() to detect
  errors.

The new blkdev_get_*() functions are with proper docbook comments.
While at it, add function description to blkdev_get() too.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Philipp Reisner <[email protected]>
Cc: Neil Brown <[email protected]>
Cc: Mike Snitzer <[email protected]>
Cc: Joern Engel <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: KONISHI Ryusuke <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Alexander Viro <[email protected]>

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