fatfs: fix build warning with 64k PAGE_SIZE

Annoying gcc warning:

fs/fat/inode.c: In function 'fat_fill_super':
fs/fat/inode.c:1222: warning: comparison is always false due to limited range of data type

Change it to compare with 4K instead of PAGE_CACHE_SIZE, as suggested
by OGAWA-san.

[FAT spec says: logical_sector_size should be 512, 1024, 2048 4096]
So, at least for now, we limit it to 4096.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>