os/FileStore: fix ENOENT error code for getattrs()

In commit dc0dfb9e01d593afdd430ca776cf4da2c2240a20 the omap xattrs code
moved up a block and r was no longer local to the block.  Translate
ENOENT -> 0 to compensate.

Fix the same error in _rmattrs().

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 6da4b91c07878e07f23eee563cf1d2422f348c2f)