writeback: Export __inode_attach_wb()

writeback: Export __inode_attach_wb()

Commit c26339e1df33 ("ext4: Fix data integrity writeout issues in
nojournal mode") made ext4_mark_iloc_dirty() attach the inode to a wb
before marking it for metadata writeback in nojournal mode. This is the
first modular caller of inode_attach_wb() - all users of
__inode_attach_wb() so far were built-in - so with CONFIG_EXT4_FS=m and
CONFIG_CGROUP_WRITEBACK=y the build now fails at the modpost stage:

  ERROR: modpost: "__inode_attach_wb" [fs/ext4/ext4.ko] undefined!

Export the symbol. Use EXPORT_SYMBOL_GPL() to match the other cgroup
writeback exports in this file.

Fixes: c26339e1df33 ("ext4: Fix data integrity writeout issues in nojournal mode")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607281811.F3c6kRvX-lkp@intel.com/
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>