btrfs: split btrfs_is_fstree() into multiple if statements for readability

Instead of a single if statement with multiple conditions, split it into
several if statements testing only one condition at a time and return true
or false immediately after. This makes it more immediate to reason.

The module's text size also slightly decreases, at least with gcc 14.2.0
on x86_64.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1897138	 161583	  16136	2074857	 1fa8e9	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1896976	 161583	  16136	2074695	 1fa847	fs/btrfs/btrfs.ko

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