bdi: Remove "inline" keyword from exported I_BDEV() implementation

With gcc 3.4.6/4.1.2/4.2.4 (not with 4.4.7/4.6.4/4.8.4):

      CC      fs/block_dev.o
    include/linux/fs.h:804: warning: ‘I_BDEV’ declared inline after being called
    include/linux/fs.h:804: warning: previous declaration of ‘I_BDEV’ was here

Commit a212b105 ("bdi: make inode_to_bdi() inline") added a
caller of I_BDEV() in a header file, exposing the bogus "inline" on the
exported implementation.

Drop the "inline" keyword to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jens Axboe <axboe@fb.com>