ext4: convert extents KUnit test to sget_fc()

ext4: convert extents KUnit test to sget_fc()

The extents KUnit test uses sget() to get an initialized superblock for
its fake file_system_type. sget() predates fs_context and we want to
retire it. Switch this caller over to sget_fc().

Add a no-op ext_init_fs_context() so fs_context_for_mount() has
something to call on the fake fs_type. ext_set() now takes a struct
fs_context * (still a no-op). extents_kunit_init() allocates the fc,
hands it to sget_fc() and drops the fc reference once the sb is
published. sget_fc() does not retain a pointer to it.

No functional change for the test.

Link: https://patch.msgid.link/20260529-work-sget-v2-1-57bbe08604e4@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>