fs: convert __generic_file_fsync to use errseq_t based reporting

Many simple, block-based filesystems use generic_file_fsync as their
fsync operation. Some others (ext* and fat) also call this function
to handle syncing out data.

Switch this code over to use errseq_t based error reporting so that
all of these filesystems get reliable error reporting via fsync,
fdatasync and msync.

Signed-off-by: Jeff Layton <jlayton@redhat.com>