xfs: remove dead internal transaction path from xfs_iomap_write_direct

Now that all callers supply a valid transaction via args->tp,
remove the dead internal transaction allocation path from
xfs_iomap_write_direct().

The function is now a pure allocation helper: it reserves blocks
via xfs_trans_reserve_more_inode(), extends the inode extent count,
calls xfs_bmapi_write(), and fills in the iomap. All error paths
return directly since the caller manages the transaction and lock
lifecycle.

The xfs_qm_dqattach() call is replaced with xfs_qm_dqattach_locked()
since the ILOCK is always held on entry.

Assisted-by: LLM
Signed-off-by: Dave Chinner <dgc@kernel.org>