[LIBS] vfs: ext4: Fix recently introduced bug in block and inode allocation

Looks like I screwed-up group mutex locking in my previous patch.

This patch fixes group mutext locking in ext4fs_control_alloc_block()
and ext4fs_control_alloc_inode()

Sorry for the noise.

Signed-off-by: Anup Patel <anup@brainfault.org>