fat: create a function to calculate the timezone offest

Patch series "fat: timestamp updates", v5.

fat/msdos timestamps are stored on-disk with several different
granularities, some of them lower resolution than timespec64_trunc() can
provide.  In addition, they are only truncated as they are written to
disk, so the timestamps in-memory for new or modified files/directories
may be different from the same timestamps after a remount, as the
now-truncated times are re-read from the on-disk format.

These patches allow finer granularity for the timestamps where possible
and add fat-specific ->update_time inode operation and fat_truncate_time
functions to truncate each timestamp correctly, giving consistent times
across remounts.

This patch (of 4):

Move the calculation of the number of seconds in the timezone offset to a
common function.

Link: http://lkml.kernel.org/r/3671ff8cff5eeedbb85ebda5e4de0728920db4f6.1538363961.git.sorenson@redhat.com
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>