vfs: actually return fs errors from ->sync_fs

While auditing the VFS code, I noticed that while ->sync_fs is allowed
to return error codes to reflect some sort of internal filesystem error,
none of the callers actually check the return value.  Back when this
callout was introduced for sync_filesystem in 2.5 this didn't matter
because we only did it prior to rebooting, and best-effort was enough.

Nowadays, we've grown other callers that mandate persistence of fs
metadata, like syncfs(2) and quota sync.  Reporting internal fs errors
is critical for these functions, and we drop the errors.  Fix them, and
also fix FIFREEZE, so that userspace can't freeze broken filesystems.
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