logfs: fix logfs build errors and dependencies

Fix build errors that happen when CONFIG_LOGFS=y and CONFIG_MTD=m:

  fs/built-in.o: In function `logfs_mount':
  super.c:(.text+0x92a6f): undefined reference to `logfs_get_sb_mtd'
  fs/built-in.o: In function `logfs_get_sb_bdev':
  (.text+0x93530): undefined reference to `logfs_get_sb_mtd'

This patch avoids the error by changing the dependencies of logfs in a
way that we can no longer configure logfs as built-in when the MTD core
is a loadable module, while leaving the dependency to require at least
one of MTD or BLOCK to be enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Peter Chen <peter.chen@freescale.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Joern Engel <joern@logfs.org>
Cc: Prasad Joshi <prasadjoshi.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>