cmds/core/fusermount: restore privs correctly
cmds/core/fusermount: restore privs correctly

Setfsuid/Setfsgid from x/sys/unix don't return the previous filesystem
uid/gid, so restorePrivs will be a no-op as it is just setting previous
fileSystemUID and fileSystemGID again.

golang/sys@e047566fdf introduced the SetfsuidRetUid/SetfsgidRetGid
wrappers for the setfsuid/setfsgid syscalls returning the previous
fileystem uid/gid. Use these and store the previous values, so they can
be used in restorePrivs.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

hugelgupf/u-root,GanShun/u-root,u-root/u-root,u-root/u-root,GanShun/u-root,GanShun/u-root,hugelgupf/u-root,hugelgupf/u-root,hugelgupf/u-root,u-root/u-root