afs: Don't clear afs_CacheTooFull prematurely

Currently, we can clear afs_CacheTooFull here, even if
afs_CacheIsTooFull() doesn't agree that the cache is no longer 'too
full'. This could theoretically result in afs_CacheTooFull being
cleared, even though the cache is indeed 'too full', according to
afs_CacheIsTooFull(). Just break here, and let afs_CacheIsTooFull()
decide.

This reverts a small part of 488c7c97854a4bd0ec67bcfe17df93b3fd025f88.
This part doesn't seem important to the functionality in that commit,
though; the rest of that commit is still here, and avoids the extra
work if we have calculated no needed space to free.

Change-Id: If0adce7fa2e88a970ddb202de02c8ff048d2ad15
Reviewed-on: http://gerrit.openafs.org/10460
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>