Btrfs: don't store NULL byte in symlink extents

It is not necessary to store the NULL byte in a symlink inline file
extent. There's currently no code that requires the NULL byte to be
present in the extent. This change also doesn't break file format
compatibility nor the send/receive feature.

The VFS also doesn't need the NULL byte to be present in the extent,
as it reads up to inode->i_size bytes (which already excluded the NULL
byte) and sets the NULL byte for us (in fs/namei.c:page_getlink()).

So with this change we save 1 byte per symlink file extent (which is
always inlined in the btree leaf) without losing backward and forward
compatibility.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>