Tighten up FS.mountable().
Tighten up FS.mountable().

Only calculate values if needed and omit redundant startswith check.

Add a comment to clarify middle loop's purpose.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

AdamWill/blivet,rvykydal/blivet,AdamWill/blivet,rhinstaller/blivet,rvykydal/blivet,jkonecny12/blivet,dwlehman/blivet,vpodzime/blivet,dwlehman/blivet,jkonecny12/blivet,vpodzime/blivet,vojtechtrefny/blivet,rhinstaller/blivet,vojtechtrefny/blivet