xfs: fix memory leak in xfs_iext_free_last_leaf

found the issue by kmemleak.
unreferenced object 0xffff8800674611c0 (size 16):
    xfs_iext_insert+0x82a/0xa90 [xfs]
    xfs_bmap_add_extent_hole_delay+0x1e5/0x5b0 [xfs]
    xfs_bmapi_reserve_delalloc+0x483/0x530 [xfs]
    xfs_file_iomap_begin+0xac8/0xd40 [xfs]
    iomap_apply+0xb8/0x1b0
    iomap_file_buffered_write+0xac/0xe0
    xfs_file_buffered_aio_write+0x198/0x420 [xfs]
    xfs_file_write_iter+0x23f/0x2a0 [xfs]
    __vfs_write+0x23e/0x340
    vfs_write+0xe9/0x240
    SyS_write+0xa1/0x120
    do_syscall_64+0xda/0x260

Signed-off-by: Shu Wang <shuwang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>