btrfs: make btrfs_is_subpage() to return false directly for 4K page size

Btrfs only supports sectorsize 4K, 8K, 16K, 32K, 64K for now, thus for
systems with 4K page size, there is no way the fs is subpage (sectorsize
< PAGE_SIZE).

So here we define btrfs_is_subpage() different according to the
PAGE_SIZE:

- PAGE_SIZE > 4K
  We may hit real subpage cases, define btrfs_is_subpage() as a regular
  function and do the usual checks.

- PAGE_SIZE == 4K (no smaller PAGE_SIZE support AFAIK)
  There is no way the fs is subpage, so just define btrfs_is_subpage()
  as an inline function which always return false.

This saves about 7K bytes for x86_64 debug builds:

	   text	   data	    bss	    dec	    hex	filename
Before:	1484452	 168693	  25776	1678921	 199e49	fs/btrfs/btrfs.ko
After:	1476605	 168445	  25776	1670826	 197eaa	fs/btrfs/btrfs.ko

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>