fuse2fs: refactor uid/gid setting

Don't open-code the uid and gid update logic.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/175182663023.1984706.1635443008190304976.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>