ext4: hoist ext4_block_write_begin and replace the __block_write_begin

ext4: hoist ext4_block_write_begin and replace the __block_write_begin

Using __block_write_begin() make it inconvenient to journal the
user data dirty process. We can't tell the block layer maintainer,
‘Hey, we want to trace the dirty user data in ext4, can we add some
special code for ext4 in __block_write_begin?’:P

So use ext4_block_write_begin() instead.

The two functions are basically doing the same thing except for the
fscrypt related code. Remove the unnecessary #ifdef since
fscrypt_inode_uses_fs_layer_crypto() returns false (and it's known at
compile time) when !CONFIG_FS_ENCRYPTION.

And hoist the ext4_block_write_begin so that it can be used in other
files.

Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240830053739.3588573-3-zhangshida@kylinos.cn
Signed-off-by: Theodore Ts'o <tytso@mit.edu>