linux2.6.38之後內核版本調用open

linux2.6.38之後內核版本下編譯內核模塊，當調用open_by_devnum()函數時會出錯。該問題原因在於：在linux2.6.37版本以後的內核中，已將open_by_devnum()函數去除，而用 blkdev_get_by_dev函數取代其功能。

可參考如下引用內容：

xen/blkback: Update to use blkdev_get_by_dev instead of open_by_devnum.

Subject: [PATCH 7/5] block: clean up blkdev_get() wrappers and their users
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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.