btrfs: add optimized btrfs_ino() version for 64 bits systems

Currently btrfs_ino() tries to use first the objectid of the inode's
location key. This is to avoid truncation of the inode number on 32 bits
platforms because the i_ino field of struct inode has the unsigned long
type, while the objectid is a 64 bits unsigned type (u64) on every system.
This logic was added in commit 33345d01 ("Btrfs: Always use 64bit
inode number").

However if we are running on a 64 bits system, we can always directly
return the i_ino value from struct inode, which eliminates the need for
he special if statement that tests for a location key type of
BTRFS_ROOT_ITEM_KEY - in which case i_ino may not have the same value as
the objectid in the inode's location objectid, it may have a value of
BTRFS_EMPTY_SUBVOL_DIR_OBJECTID, for the case of snapshots of trees with
subvolumes/snapshots inside them.

So add a special version for 64 bits system that directly returns i_ino
of struct inode. This eliminates one branch and reduces the overall code
size, since btrfs_ino() is an inline function that is extensively used.

Before:

$ size fs/btrfs/btrfs.ko
   text	   data	    bss	    dec	    hex	filename
1617487	 189240	  29032	1835759	 1c02ef	fs/btrfs/btrfs.ko

After:

$ size fs/btrfs/btrfs.ko
   text	   data	    bss	    dec	    hex	filename
1612028	 189180	  29032	1830240	 1bed60	fs/btrfs/btrfs.ko

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>