ext4: avoid setting directory i_nlink to zero

If a directory with more than EXT4_LINK_MAX subdirectories, the nlink
count is set to 1.  Subsequently, if any subdirectories are deleted,
ext4_dec_count() decrements the i_nlink count, which may go to 0
temporarily before being incremented back to 1.

While this is done under i_mutex, which prevents races for directory
and inode operations that check i_nlink, the temporary i_nlink == 0
case is exposed to userspace via stat() and similar calls that do not
hold i_mutex.

Instead, change the code to not decrement i_nlink count for any
directories that do not already have i_nlink larger than 2.

Reported-by: Cliff White <cliffw@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>