statvfs.3: note f_favail = f_ffree on Linux

statvfs.3: note f_favail = f_ffree on Linux

Quoting myself from #musl:
01:59:40 hm, I think this was just invented for symmetry with bfree/bavail
02:00:46 FFS has minfree for space but nothing equivalent for inodes
02:32:31 (this is mirrored in ext4;
	  a global grep over DragonFlyBSD and the illumos gate
	  showed just NFSv3 forwarding from the server;
	  OpenBSD always sets it to the same thing as f_ffree;
	  oddly, NetBSD /does/ calculate it differently
	  for LFS and FFS but due to queued writes or
	  w/e not because of root reservation;
	  and as expected a lot of "/* what to put in here? */"
	                       and "// XXX same??")

Link: https://lore.kernel.org/linux-man/f54kudgblgk643u32tb6at4cd3kkzha6hslahv24szs4raroaz@ogivjbfdaqtb/t/#u
Cc: Tom Schwindl <schwindl@posteo.de>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx@kernel.org>