Fix the following warnings:

fs/udf/balloc.c:768:15: warning: conversion to 'sector_t' from 'int'
may change the sign of the result [-Wsign-conversion]
   allocated = udf_bitmap_prealloc_blocks(sb,
               ^
fs/udf/balloc.c:773:15: warning: conversion to 'sector_t' from 'int'
may change the sign of the result [-Wsign-conversion]
   allocated = udf_table_prealloc_blocks(sb,
               ^
fs/udf/balloc.c:778:15: warning: conversion to 'sector_t' from 'int'
may change the sign of the result [-Wsign-conversion]
   allocated = udf_bitmap_prealloc_blocks(sb,
               ^
fs/udf/balloc.c:783:15: warning: conversion to 'sector_t' from 'int'
may change the sign of the result [-Wsign-conversion]
   allocated = udf_table_prealloc_blocks(sb,
               ^
fs/udf/balloc.c:791:26: warning: conversion to 'loff_t' from 'sector_t'
may change the sign of the result [-Wsign-conversion]
   inode_add_bytes(inode, allocated << sb->s_blocksize_bits);
                          ^
fs/udf/balloc.c:792:2: warning: conversion to 'int' from 'sector_t'
may alter its value [-Wconversion]
  return allocated;
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jan Kara <jack@suse.cz>