that space for extended attributes should be preallocated, instead of
using a sparse attribute file. NOTE: This can result in a really
large file full of zeros. However, it can prevent a low disk condition
from causing an attribute write to fail, which is good for security and
consistency attributes.
o Unlink the attribute file during initattr if an error occurs -- this is
alright, as we specify O_CREAT when opening the file.
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