os/FileStore: automatically enable 'filestore xattr use omap' as needed

os/FileStore: automatically enable 'filestore xattr use omap' as needed

Automatically enable the 'filestore xattr use omap' option if the fs
does not appear to handle large xattrs on its own.

This makes for a more pleasant use experience as they are not told to
enable something that we already know they must enable in order to
continue.

Fixes: #5137
Signed-off-by: Sage Weil <sage@inktank.com>