jbd2: use gfp_t instead of int

This silences some Sparse warnings:
fs/jbd2/transaction.c:135:69: warning: incorrect type in argument 2 (different base types)
fs/jbd2/transaction.c:135:69:    expected restricted gfp_t [usertype] flags
fs/jbd2/transaction.c:135:69:    got int [signed] gfp_mask

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>