fuse2fs: set fuse subtype via argv[0] if possible

fuse2fs: set fuse subtype via argv[0] if possible

If argv[0] ends in "ext[0-9]", set the fuse subtype string to this
value.  This enables us to place fuse2fs at some place in the filesystem
like /sbin/mount.ext2 and have /proc/mounts report the filesystem type
as "fuse.ext2".  This is fairly boring, but it'll make it easier to test
things in fstests.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/174553064709.1160289.11028230202411857669.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>