Before you set out to farm linen cloth, check the auction house. Linen is often sold for coppers a piece, usually making it much more worthwhile to simply buy it than farm it. But depending on the server, the quantity might not be enough, especially if you are planning to craft a ton of items, such as shirts, for transmog purposes.
For Alliance, Westfall has near instant respawns of Riverpaw Gnolls. However, if you have done Loremaster in Westfall, these mobs might be phased out. But it is just a short flight from Stormwind to check.
Deadmines is a great place to farm for linen cloth. Simply pull all the mobs to each boss and AOE. You will also get some wool cloth too.