libfiemap: Only call FS_IOC_FIEMAP once.

The kernel can return different compatible ranges on each call, depending on
how it decides to merge contiguous extents in the results. To avoid the
complexity of requerying the ioctl, just do one query up to the maximum
allowed extent size.

Bug: 204536075
Test: install DSU on cuttlefish
      fiemap_test
Change-Id: I4d569e3e6feed14c91a5f500296623888060dcad