[XFS] Fix char size overflow in bmap_alloc call for unwritten extent

conversion.

Since bma.conv is a char and XFS_BMAPI_CONVERT is 0x1000, bma.conv was
always assigned zero. Spotted by the GNU C compiler (SVN version).

SGI-PV: 947312
SGI-Modid: xfs-linux-melb:xfs-kern:26887a

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Nathan Scott <nathans@sgi.com>