fs/bfs: use bfs prefix for dump_imap

All bfs related functions use bfs_ prefix.  This patch also moves extern
declaration to bfs.h and removes prototype from inode.c

This fixes checkpatch warning:

  WARNING: externs should be avoided in .c files

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: "Tigran A. Aivazian" <tigran@aivazian.fsnet.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>