btrfs: inline btrfs_space_info_used()

btrfs: inline btrfs_space_info_used()

The function is simple enough to be inlined and in fact doing it even
reduces the object code. In x86_64 with gcc 14.2.0-19 from Debian the
results were the following:

  Before this change

    $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
    1919410	 161703	  15592	2096705	 1ffe41	fs/btrfs/btrfs.ko

  After this change

    $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
    1918991	 161675	  15592	2096258	 1ffc82	fs/btrfs/btrfs.ko

Also remove the ASSERT() that checks the space_info argument is not NULL,
as it's odd to be there since it can never be NULL and in case that ever
happens during development, a stack trace from a NULL pointer dereference
will be obvious. It was originally added when btrfs_space_info_used() was
introduced in commit 4136135b080f ("Btrfs: use helper to get used bytes
of space_info").

Also add a lockdep assertion to check the space_info's lock is being held
by the calling task.

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>