drivers/char/mem.c: memory_open() cleanup: lookup minor device number from devlist

memory_open() ignores devlist and does a switch for each item, duplicating
code and conditional definitions.

Clean it up by adding backing_dev_info to devlist and use it to lookup for
the minor device.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Adriano dos Santos Fernandes <adrianosf@uol.com.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>