fat: fix using uninitialized fields of fat_inode/fsinfo_inode

fat: fix using uninitialized fields of fat_inode/fsinfo_inode

Recently fallocate patch was merged and it uses
MSDOS_I(inode)->mmu_private at fat_evict_inode().  However,
fat_inode/fsinfo_inode that was introduced in past didn't initialize
MSDOS_I(inode) properly.

With those combinations, it became the cause of accessing random entry
in FAT area.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: OGAWA Hirofumi <[email protected]>
Reported-by: Moreno Bartalucci <[email protected]>
Tested-by: Moreno Bartalucci <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>