ext4: optimize ext4_should_retry_alloc() to improve ENOSPC performance

If there are no pending blocks to be released after a commit, forcing
a journal commit has no hope of helping.  It's possible that a commit
had just completed, so if there are now free blocks available for
allocation, it's worth retrying the commit.

Reported-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>