xfs: make pNFS block allocation atomic with inode update

Restructure xfs_fs_map_blocks() to perform the extent allocation
and inode update in a single synchronous transaction, making the
operation atomic with respect to the ILOCK.

Previously the function used two separate transactions: one for
the extent allocation (via xfs_iomap_write_direct) and one for the
inode update (xfs_fs_map_update_inode), followed by an explicit
log force to ensure persistence.

Now the function uses a do {} while (!dwa.tp) loop to allocate a
transaction and re-read the mapping atomically. The inode update
(SUID/SGID strip, timestamps, PREALLOC flag) is performed in the
same transaction as the allocation, and the transaction is committed
synchronously via xfs_trans_set_sync().

xfs_fs_map_update_inode() is converted to take a transaction
parameter and no longer allocates or commits its own transaction.

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