ext4: refactor the inline directory conversion and new directory codepaths

ext4: refactor the inline directory conversion and new directory codepaths

There was a lot of common code in the codepaths used to convert an
inline directory and to creaet a new directory.  To address this,
rename ext4_init_dot_dotdot() to ext4_init_dirblock() and then move
common code into that function.

This reduces the lines of code count in fs/ext4/inline.c and
fs/ext4/namei.c, as well as reducing the size of their object files.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://patch.msgid.link/20250712181249.434530-3-tytso@mit.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>