sysfs: Rename struct bin_attribute member to f_mapping

There are two users of iomem_get_mapping(), the struct file and struct
bin_attribute.  The former has a member called "f_mapping" and the
latter has a member called "mapping", and both are poniters to struct
address_space.

Rename struct bin_attribute member to "f_mapping" to keep both meaning
and the usage consistent with other users of iomem_get_mapping().

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Link: https://lore.kernel.org/r/20210729233235.1508920-3-kw@linux.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>