btrfs: move fiemap code into its own file

Currently the core of the fiemap code lives in extent_io.c, which does
not make any sense because it's not related to extent IO at all (and it
was not as well before the big rewrite of fiemap I did some time ago).
The entry point for fiemap, btrfs_fiemap(), lives in inode.c since it's
an inode operation.

Since there's a significant amount of fiemap code, move all of it into a
dedicated file, including its entry point inode.c:btrfs_fiemap().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>