fat: move fat_i_pos_read to fat.h

fat: move fat_i_pos_read to fat.h

Originally, fat_i_pos_read() was a static inline function in inode.c.  The
static keyword was removed so that it could be accessed from other files.
However it is better to make it a static inline function in fat.h as
suggested by Ogawa.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ravishankar N <ravi.n1@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>