fat: eliminate iterations in fat_search_long and __fat_readdir in case of EOD

fat: eliminate iterations in fat_search_long and __fat_readdir in case of EOD

When doing lookups via fat_search_long(), we can stop checking for further
entries if we detect End of Directory, i.e.  if (de->name[0] == 0x00).The
current code traverses the cluster chain of a directory until a hit is
found or till the last cluster for that directory, ignoring the EOD mark.
Fix this.

Likewise,when readdir(3) is called, we can stop checking for further
entries in __fat_readdir() when we hit EOD.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ravishankar N <ravi.n1@samsung.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>