mbcache: revert "fs/mbcache.c: make count_objects() more robust"

This reverts commit d5dabd63.

This patch did absolutely nothing, because ->c_entry_count is unsigned.

In addition if there is a bug in how mbcache maintains its entry count,
it needs to be fixed, not just hacked around.  (There is no obvious bug,
though.)

Cc: Jan Kara <jack@suse.cz>
Cc: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>