fs: rename generic_delete_inode() and generic_drop_inode()

fs: rename generic_delete_inode() and generic_drop_inode()

generic_delete_inode() is rather misleading for what the routine is
doing. inode_just_drop() should be much clearer.

The new naming is inconsistent with generic_drop_inode(), so rename that
one as well with inode_ as the suffix.

No functional changes.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>