fs: fixup warning part_discard_alignment_show()

Stephen reports:

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After merging the block tree, today's linux-next build (x86_64
allmodconfig) produced this warning:

fs/partitions/check.c: In function 'part_discard_alignment_show':
fs/partitions/check.c:263: warning: format '%u' expects type 'unsigned int', but argument 3 has type 'long long unsigned int'

Introduced by commit  ("block: Remove extra discard_alignment from
hd_struct")

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Fix it up by just removing the cast, we return an int already.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>