os/FileStore: fix max object name limit

Our max object name is not limited by file name size, but by the length of
the name we can stuff in an xattr.  That will vary from file system to
file system, so just make this 4096.  In practice, it should be limited
via the global tunable, if it is adjusted at all.

Signed-off-by: Sage Weil <sage@redhat.com>